I've always suspected / assumed that was WotC wanting to keep things much more on the casual / newer player side of the line to make sure they have the furthest reach to all players... knowing full well there would be other designers / companies willing to take on the biggest task of making more complexity for those tables that want it, even though the audience is going to be smaller. As
@Morrus, Kobold Press and MCDM are doing.
The only downside to this of course being those players who want more complexity but still have a bugaboo about using anything "not official". Those that think that anything not made by WotC is therefore not playtested and unbalanced... so they just sit on their hands waiting for WotC to get around to it rather than using something already made that would serve their purposes.