Cost, your average stinger cost 38000 dollars in the 80s. I am guessing here but they are aiming at a price of probably under 10000 dollars.

Now why doesn't anyone take a rocket and stick the drone guidance on it? Again I am only guessing here, the drone guidance components probably can't cope with 2-3 mach. At 1000 meters per second with a 60 fps camera you advance 16.6 meters per frame, add to that the latency of whatever guidance system you have. You are looking at 20-30 meters offset between frames.

Better guidance probably balloons the cost to the 10s of thousands of dollars.

Oh and probably loiter time, the rocket is you shoot and it's gone. The interceptor may have enough battery to fly around for a few minutes looking for the next one if its target is downed by something else. Plus you can probably recover these if they miss. This is all very speculative though.