Following the theory of system mastery, the game in theory cannot be simple. There needs to be built-in exceptions, tricks, little twists and turns of rules, gotchas, and I-don't-know moments of vague presentation that require a college level of mastery of a particular rules set. The more arcane, the better, because this allows the 'player expert' to become more valuable to a particular group or setting.
Anthropology? You feel like a scientist digging through the history of these games to just be able to play them. They are incompatible with each other, have enough small changes between them that one game is realistically all an average person can play, and some of the assumptions you carried over from previous editions will trip you up. It's why I still like Labyrinth Lord or Basic Fantasy as my go-to D&D games, they stick to the core design, there isn't much to learn, and there isn't much to mess you up.
Self-Taught ExpertsThat "power of knowledge" is an important thing. It both creates a barrier to entry (noobs suck at the game), and it creates a barrier to exit (why would I give up all this knowledge and power). It gives those who invest the time a certain level of importance in communities, power over others, and value. If the game is widely played, the more value that person has in a community, gaming group, or forum. Also, if the books needed are expensive and voluminous, the investment in the game keeps people from leaving as well. No one wants to have wasted all that money.
You can short-circuit all of this and say, "I don't have the time," and walk out. You can go to a simple rules system. Many modern designs cater to people without the time to learn all this complexity, and these keep today's time and interest-strapped people in the hobby. They are also exit-points for people that are tired of the heavy games, as a group can only really maintain interest in one large game at a time.
Some modern non-d20 games follow the system mastery model as well, such as Fantasy Flight's Star Wars RPG. While the basic mechanic is simple (as presented in the starter sets), once you go all-in with the main rulebooks there are enough gotchas where you need an expert to sort things out. This is similar for Shadowrun, GURPS, and other games where the weight of expansions and rulings have turned a once-simple game into something more complex.
Games do not need to be that complex. Complexity is not depth. A simple game can have wonderful depth without complexity, take chess, for example. This is where great design comes into play. Some designers seem to give up on designing elegant, simple, yet very deep in strategy rules, and go for the arcane, complex, and exception-filled almost spaghetti computer-code level of complexity when depth by the base mechanic cannot be achieved. The depth is obfuscated in the complexity of the rules.
Modern DesignsSome games are by-design simple, and this applies to the OGL retro-clones, Savage Worlds, Legend, AGE System, FATE, and many modern designs. These games have simple rules of thumb, unified mechanics, and a simple set of core rules which are extended to handle anything. If you know how to do one thing in these games, you can assume many other things work similarly and you are on your way. Some players love the elegance of unified design simplicity, while others love system mastery and the power of that complex knowledge.
A lot of indie games embrace simplicity, because it is a lot easier to test and balance a small set of rules-of-thumb than it is an encyclopedia of specific and carefully-balanced rules. I feel the age of huge, complex systems is passing us by, as even D&D 5 has recognized the need to simplify to increase "what can get done" at the table during a session. They streamlined and simplified monster stats, character complexity, and the amount of modifiers and conditions you needed to remember, and players reacted positively to the changes. As a result, the games move faster, players can cycle through more encounters during a session, and a game session can end with a feeling of progress and completion in regards to a story.
Savage Worlds goes in a different direction than D&D 5, with combat meaning less and all actions taking on an equal level of importance in regards to story completion and advancement. Conversions of modules from Pathfinder, OGL games, and D&D 5 to Savage Worlds typically eliminate encounters meant to "level up" the party and focus more on tasks, social encounters, and roleplaying. D&D 5, in comparison, is the more 'video game' and 'XP and loot' type experience, where Savage Worlds is the more the story-based experience.
There Comes a Point...When you don't have the time to invest in heavy games anymore. It is happening to us, and I have this feeling Bestiary 5 and Ultimate Intrigue will be the last Pathfinder books we will buy for a while. I love the world, I love the mod-able-ness, I love the art, and I love the system, but enough is enough. I can't run this much, and I can't manage this much complexity anymore. If I play, it will be with a subset of books, but even then I question if investing myself in a reduced subset of that complexity will be worth the payout. Learning and mastering a simpler game at this point may give us more bang for the gaming buck.
Even the new Star Wars RPG still feels bloated to us, and we've reduced the game to the base three books. It is the problem with everything being a heavy game, you want to play them all, but one person really can't master and maintain interest in this many large games. They makes these games complex with the theory of system mastery so you won't leave for another game, and then when you collect too many of them, everything in your gaming world collapses under the weight of complexity.
OGL games are cool, and they "get there from here" in terms of providing a dungeon experience for the complexity and time spent learning them. Savage Worlds is also cool and simple, and covers so many genres that it is worth learning because the payout in "fun for time" is very high. D&D 5 is interesting, but we haven't had the time or interest to put into it as much as I'd hoped, plus the OGL games do a good job at the dungeon and loot experience anyways so we haven't had much pressure to jump in.
In our group and for our time, Pathfinder, Shadowrun, and the new Star Wars RPG feel like the losers here. We don't have the time to put into and enjoy them as we would like, and something has to give. When you need to make choices, you typically start throwing the heavy systems overboard first (or you stick with one heavy system and throw out everything else). This isn't to say they aren't deep and fun games, it is just the complexity and time needed to put in to enjoy them fully is very high.
I will probably reorganize my shelves to reflect the new order, and focus on what is right for our group. This is also a consideration when you design a game, do you want it to be so complex that is pushes out other systems, or do you want it to be rules-light and easily accessible? I don't think many games nowadays can compete with the big heavy-hitters out there, so writing a large and complicated game will put you at a disadvantage of competing for attention of players and also being pushed off a shelf due to the heavy nature of your rules.
We do have a lot of large, complicated games sitting in boxes because we don't have the time or mental space to run them (Rifts, GURPS, and others). I feel bad about a couple more being on their way to storage, but really, what can we do? It is best to focus on the games you love, and the ones you have time for, rather than so many you are unhappy you don't have the time and mental effort to play them all.