rabbiofberlin: Because it adds quite a bit of time to davening, and many people aren’t interested in a drawn out davening. It’s one thing if there’s a good baal tefilah, but for a Carlebach davening, they’re not interested.
Personally, I’m not a Carlebach fan. Many of his slow songs sound like funeral marches, and most of his songs sound the same.
When I was in yeshiva, there was some debate as to whether guys should listen to Carlebach. R’ Moshe’s teshuvah in the Igros on it was brought up. One Rebbe in the yeshiva told us that his Rebbe, R’ Mendel Kaplan zt”l had addressed the issue years before. He said that halachicly, there is no issur to listen to Carlebach music (except for the albums where he has women singing), but that listening to his music would cause you to get depressed.