The subs that are diverse are small enough that they never get seen by the majority of users. If they were large enough to be seen by the majority then they would cease to be diverse. The Reddit approach of subverses is the right idea, however when a subverse reaches a certain size it creates users akin to cultists. These users then infect other subverses like pathogens. The process is remarkably similar to how viruses operate, invading the host cell and producing copies of the virus before the cell bursts and the virus invades neighboring cells. Diverse ideas are not capable of reproduction at the rate of viral ideas. A few viral ideas, the simpler the better, prove to be effective at rapidly reproducing within the current environment. They sprout up like weeds and outcompete deep rooted and slow reproducing ideas. When you reach a critical mass of weeds you lose another space to the better adapted invasive species.
Wouldn't reddit be considered big and diverse? You still have T_D and mensrights and a bunch of other conservative subs. True, on a sub by sub basis, the mods and voters tend to enforce one ethos, but there are a few subs with substantial left and right participation. Politicaldiscussion comes to mind.