Because it still isn't cost effective. A drone that can be guided, either autonomously or by a human, will cost at least, what, $80 wholesale? That will almost equal the price of the ordinance. Causing a lot of casualties at a low cost is one of the major appeals of field artillery and explosives.

Now, maybe it would make more sense for the assassination of carefully guarded targets. But it will probably be a few more years before that technology is more reliable than strapping a vest containing c4 on someone.