Smart people are working on all this, so I'm sure there are reasons, but in the all the instances where I've had to interact with a monorepo, it was because the tech debt was too high to pay off to break it apart, not because it was better.
And if you're indebted to the point where you have no point in paying it off, you damn well better have leveraged assets against it (i.e. a cash cow of a business, like MS Word or Facebook)
As far as Git's killer feature I think decentralization is only one part of it. Being able to deal with the change graph directly is sometimes handy. And knowing the basis of the model, I find Git pretty easy to understand even when history gets fuzzy. Not sure but I think Darcs might be even better for that, just without the mindshare.
Why spend so much effort bashing a distributed peg into a centralized hole?