Just because they call it a monoculture, doesn't mean it has anything to do with social behavior per se. It's a technical choice. Might as well ask why every function isn't implemented in X languages or X styles. Using a singular tool in a build is the default. It's a surprise to me, that they still were trying to support a bunch of disparate compilers. Let someone else handle that, while you run the race of feature implementation, which is hard enough.
That said, I think I agree with your overall point. With the amusing counter that most new languages do go way the hell out of their way to start implementing everything. In large to keep you in that language. Even in the web services world, it amuses me to see so many folks try and cram everything into a single service where they can.
The flip side, of course, is you do have a finite budge of energy to make things happen. It makes little sense to spend that budget solving problems you don't have. Worse, you could rathole into solving technical challenges that you may never actually have a need to solve. All the while leaving your customers dry.
So, yes, there is a tradeoff. I fully agree with that. Which is why this topic is interesting to me. All of the incentives seem to be to embrace monoculture thinking in most that you do. I can't help but think that is off in the long view, though. :(