For the same reason we don't use horse and buggy anymore (shout out to the Amish breaking tradition and using a computer to read this).
Does it work? Yes. Will it get ya where you need to go? Yes. Is it cheap on gas and really unlikely to get carjacked? Yes and yes.
But, daddy doesn't want to take 2 hours to go the 3 miles for a Dunkin run.
Advancements in ammo over the last 40 years makes the 9mm/45ACP debate less and less worthwhile. But, the .32 to modern 9mm is less a debate.
As to why it was acceptable when the 45ACP was around, I'm guessing cost per round was the first reason. Then recoil and finally since the .32 came out 5 or 6 years before the 45ACP, if LEO/military were already carrying a pistol in that caliber, it wasn't financially prudent to replace to the 45ACP, especially if they were personally paying out of pocket.