Because everyone likes 9mm these days, for good reason.
Because SOME shooters like 9mm these days, for SOME reason, and think that they are "everyone" for NO clear reason whatsoever.
I own, carry, and prefer a .45 ACP. I own, shoot, and almost NEVER carry a 9mm (but someone I love DOES). If I could not have a .45, I would opt for a .40 S&W or a 10mm. If I could not have one of those, I would likely be restricted to 9mm, which is why I own one.
Perhaps this has been mentioned before, but most pistols originally made in 9mm could also be manufactured in .40 S&W (Glock, SiG, S&W, Ruger, Cz, Browning, 1911A1, doo-dah, doo-dah...), without major re-engineering. This offers a significant increase in power over the 9mm, with no real change in ergonomics, and a minor decrease in magazine capacity.
One of the reasons I don't own a Browning "Hi-Power" (a term I use with tongue planted firmly in cheek) is, for the longest time, it was offered only in 9mm, and I didn't have the kind of discretionary income that would allow me to spend that much for a pistol I would only use occasionally. If the pistol could have been chambered for .45 ACP, I would have likely foregone my interest in the the 1911A1, and bought a Browning, straight away. Toward the end of its production career, it was made in .40 S&W, and I moved heaven and earth to get one, but never succeeded.
I PREFER .45 ACP over the .40 S&W, but I don't dislike the latter. For that matter, I don't dislike the 9mm. I just prefer to trust my life to sidearms that throw larger projectiles.
If someone tells me that a 9mm is superior to a .45 ACP as a cary weapon, they COULD be right, if the weapon is in THEIR HANDS. If I tell someone that a .45 ACP is superior to a 9mm, I AM right, if the weapon is in MY HANDS. If someone who cannot connect with a .45 can hit splendidly with a 9mm, they should probably carry a 9mm. But I'll wager that half of those shooters could shoot a weapon chambered for .40 S&W almost as well as they could a 9mm, and with far more authority on the business end. Ulitmately, one should carry the weapon with which they shoot best, and are most comfortable in carrying.