Because they work with a tried and true design that works. Speaking of design, there are lots of custom options out there for those who want it stock or love it tricked out. In an all steel version, the legendary .45 acp is also a joy to shoot.
The design is in public domain too so lots of people are making them at various
I also think the gun itself has so much American history behind it that it's hard not to like the 1911.
"He who laughs last, laughs dead." Homer Simpson