Because Git is used by Linux and was designed by Linus.
Those facts alone are good enough for most people to choose git over something else, not the technical ones.
Just like UNIX and JavaScript, technically better is not always what wins.
Honestly it's a good story about the power of marketing and I feel a bit sorry for the monotone developers because they did it first and well, but get almost no recognition.
Fast?
I played with OpenEmbedded (for OpenMoko) a while ago, which used monotone. From what I recall, it was slow. IIRC, slow enough that they had to use the trick of downloading a "pre-cloned" snapshot and updating it, instead of cloning from scratch.
I did find some benchmarks from 2009 and by then monotone shows consistent by often 5x slower performance than git:
https://web.archive.org/web/20090131095353/http://ldn.linuxf...
"Worse is better" etc.
(Completely unrelated, I just discovered that the Plan 9 logo bears a striking resemblance to the Go logo [see the Wikipedia link]. Given Rob Pike's influence, I'm sure that is not a coincidence.)