What is it that I am missing???
Well, in my case, I'm not as flexible as I once was, and not as well balanced. There's a time issue too. It takes longer to bend over, raise a pant leg, and retrieve the firearm. The "pistolero experts" have frowned upon ankle carry for many years due to the long time to draw. Back in the 70's, cops would ankle carry for back-up guns, but times have changed. Heck, most of the "experts" don't like drop leg holsters either due to the draw times. Ankle carry is probably the most impractical method of carry. And if you can ankle carry a particular firearm, then you probably can pocket carry it too. I've been known to carry a NAA .22LR revolver in a neck holster.
Getting back to the sling pack, the Maxpedition versapack has a holster built into it, which is handy, but like an ankle holster, you're not going to set any draw records.