Wondering about why git became standard is a bit like wondering why mammals became big and more dominant a few tens of millions of years ago. It was a side effect of other bigger changes and being in the right place at the right time. Not that many other factors mentioned here were unimportant, but I don't think they're the high order bit.

By the early mid 2000's, Microsoft's dominance in the software space was under serious attack. The web was beginning to get to be a more serious place to build apps, undermining a lot of the market that MS had dominated for in-house apps (visual basic etc etc). Things like WHATWG, and then Safari, and eventually Chrome helped to decouple lots of apps from dependence on proprietary Windows technologies. Additionally, cloud computing was taking off in one form or another (VMware and virtualization, then actual cloud offerings like AWS), which made linux a much more appealing platform for doing a lot of server-side development. These two factors, I think, helped move unix/linux environments into more of a mainstream place in many developer's lives.

Not that they were not viable before, but endless stories about the titans of the new industry relying on commodity, cheap, open source platforms and technologies certainly influenced many organization's mindsets. (it probably doesn't hurt that the transition to .net wasn't the smoothest not to mention Windows Vista, but that's also probably not the high order bit).

Another interesting datapoint (also probably not a high order bit) is that in the mid 00's, the mac became VERY attractive among at least silicon valley developers. The presence of a linux-like development environment (Terminal etc) with a spiffy UI coupled with just being trendy (I think I'm allowed to say that, as a multi-decade  fanboy :-) ) lured many of the thought leaders away from Windows. I remember being shocked when a brand new coworker demanded a mac at work (this would have been around 2010) and actually got one... something that I'd never seen happen before. And I know of people who went to MS developer conferences in silicon valley in the same timeframe where everyone was using macs. I remember one observing: "Oh, wow. MS is in trouble".

Anyway, my own view is no one forced or made git the standard. it is more that "linux"/linux-like development environments became much, much more standard and accepted and at that point it was "the best" tool available. If MS hadn't lost its control over the software industry, lots of us might still be using visual source safe (or its successor).