I can think of a few reasons:

- 1911 triggers are generally very good and are easy to make great.

- The 1911 has a short reset and in my experience, while some pistols come close, nothing double taps better than a 1911.

- The 1911 is also comparatively thin and a Commander frame sized 1911 makes a good concealed carry handgun.

- I get 10 plus 1 rounds in my 9mm Luger 1911, compared to 8 +1 in my .45 ACP 1911. Given the improvement in 9mm Hollow point performance, I'm just as comfortable with 11 rounds of 9mm as I am with 9 rounds of .45 ACP.

- In the same amount of time it takes me score two A zone hits with .45 ACP I can score three A zone hits with 9mm Luger.

- Personally I was a die hard .45 ACP fan, until 9mm hollow point performance improved, and until I realized (or maybe just admitted t myself) that I shot 9mm Luger significantly faster at a given level of accuracy than with .45 ACP.