One advantage of the slide safety/decocker that doesn't get much mention is that you can load the chamber with the completely unable to fire because the firing pin is secured, and the pistol goes right to decock as the slide goes into battery. This is a very useful feature for military and police use.

And while we think nothing of them today, frame decockers were entirely uncommon until the mid to late '80s. Aside from the Sig P220 which came out in '75, all previous frame levers were either safeties or required the trigger to decock (Sauer 38H, P9S). AND, frame decockers are mechanically much more difficult to execute than slide decockers, meaning more complexity. Both the Sig P22X and USP pistols had their frame decockers revised to function differently early on.

Slide levers are reliable and very simple devices. Their a pain if you're going to carry safety on, but out of the way if you don't.