I love snorkels. For snorkeling. I don't wear them diving for a few reasons:
Entanglement. Kelp, fishing line, spool/reels (the latter is best typified by cavern and cave environments).
Incompatibility with hoods: When you place a snorkel on a mask strap, you limit yourself to wearing the mask strap on the outside of your wetsuit or drysuit hood. Every time I did that, the mask skirt got caught on the edge of the hood, resulting in leaks and me purging like a chump every 10-seconds. Without the snorkel, I'm free to wear to wear the mask strap on the inside of the hood, thereby eliminating one more cause of leaks (and reducing the risk of accidental mask loss.)
Confusion: I've lost count of the number of times I've seen a diver mistake a snorkel for their large-diameter inflation/deflation hose. One one or two occasions, the diver was ascending or descending out of control by the time they managed to locate the correct hose. Nothing bad happened, luckily.
Currents: Hanging on an anchor line with a roaring current = good chance that the snorkel jangling around your face will be downright annoying, if not the means for the mask to dislodge. Not fun.
They're some of the reasons I don't wear one, but if folks feel more comfortable with them, that's fine, too.