Because they kick butt! ... are accurate, reliable, easily and infinitely maintainable...
There are reasons the design has remained unchanged for 100 years!
The 1911 frame is also what caused my delay in picking up a poly frame gun... it was hard to trust anything else at first.
Selection is another... because of the age and reputation of the design it is one of the most duplicated pistols on the planet. Dozens of companies make a version of it, in multiple calibers.
Downside might be ammo capacity... with only 7 to 8 rounds per magazine. But many versions use double stack mags to achieve 10+ rounds.
I suppose the answer to the question would be, "What's not to like?"