Sharkbite and Sgt are on it. The main reason is body armor. I carried an M9 a fair bit overseas with body armor. I have carried the green flap of death. The mandate of side saapi plates pretty much mandated a drop leg holster. That being said, I hated it after I wore it on a few foot patrols. It wasnt bad at all when mounted and for short dismount operations. It is also a little more convenient when your primary weapon is a rifle and you practice transition drills. Its a little more out of the way.
That being said, if you ever have to walk more than a few hundred yards at a time a drop leg holster gets old quick. If you have to run? It gets old REAL quick. A good belt holster does the trick 99% of the time.
*Back to the military green flap holster. I dont know why but it was one of the most comfortable wearing holsters ever, as crude as it is. You wont win fastdraw competitions, but its excellent retention and comfortable to wear.