Cost, production capacity, radar cross section, speed, range, payload.

Drone means foam wings, plastic body, propellers, cheap camera, simple inertial navigation, maybe GPS, maybe 10-30 kilogram payload.

Guided missile means, metal airframe, jet engine, depending on targets thermal imaging or radar terminal guidance, radar altimeters, terrain imaging radars, 100 - 500 kilogram payload.

Remote guidance is a very hard problem, modern computers have made it much easier to solve.

Even an 80s missile, required hundred of thousands of dollars of equipment just for guidance. Now all you need is a simple computer, a cheap camera and a cheap accelerometer.

Drones are much easier to down than missiles, but they make it up in volume.