Many an argument can be made & has been made, but there are a few that will appear more often than others.
First...it's got 100 years of history built right in to it. Related to this are the fact that the world's most prolific firearms inventor/designer came up with it and the fact that it served the United States Armed Forces from the date of it's issuance until 1985. Compared to many other popular platforms, the 1911 has been around seemingly forever. This isn't lost on the movie industry either, and it's likely even more recognizable to non-gun folks than a Smith & Wesson revolver.
Second is the "feel." The grip angle is almost universally loved and all the controls feel like they were nearly perfectly designed. To many, and certainly to those who love it, no semi-auto handgun feels nearly as natural and "right" in the shooting hand as does a 1911 pistol. It's got a thin profile that is welcome in most hands.
A very close third, or perhaps it could have taken second is the extremely well designed trigger. It's a short, direct backward pull with no pivoting on any arc. If you only EVER shot 1911's from day one, there'd be a tremendously large percentage of handguns with triggers that you despise. Even some average or marginal 1911 triggers just "feel better" than so many other semi-auto handgun triggers.
Further down the list is the many years of competitive shooting and professional shooters that have made the pistol and the platform more popular than even the men & women that are the world's best shooters of it.
These are obviously subjective, but the subject matter demands it. There's also many, many other reasons and as long as someone truly believes it -- you'd have to say it's a valid reason.
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.