---
Because they fit nicely into the niche they were in. Humans and human ancestors found a new niche that was at the time unfilled, or at least open to exploitation.
---
FOUND a new niche? Evolution is blind; there's no "finding" at all. Besides, the niche is empty again. Our technology has made it obsolete to us. We should be seeing selection pressures pushing for speciation into this niche once again according to the theory.
Heck, we should be seeing selection pressure pushing into the niche REGARDLESS of whether it is filled or not. A less successful form will exist for a time, then go extinct, a more successful form will supplant what's in the niche now. Just because a niche is full doesn't mean selection suddenly stops working.
Of course, we should be seeing speciation all over the place in the biosphere (study the mathematics of continual processes and population sampling to find out why) and we don't, but try to put that in a textbook in some school somewhere and the screaming hordes of "scientists" will come out and burn you at the stake.
> FOUND a new niche?
Yes. Found a new niche. Sometimes taking a walk will put you in a whole new ecosystem where you'll prosper.
> We should be seeing selection pressures pushing for speciation into this niche once again
We have not abandoned the niches we've held for the past hundred thousand years. On the contrary... we're leading adventure safaris into them and building condos on them.
> Of course, we should be seeing speciation all over the place in the biosphere ...
Over time, yes. And that's just what the evidence shows.