Speed.

Probability of success (knowing when to kaboom).

Interceptor should be faster than the goal to have guaranteed interception and it has to have a little time to reaction (lag) to intercept it at least in best case for interceptor.

Distant detonator is also not an easy technology. BTW how many of Patriot shots are required to intercept one goal? Probably it is so because they tend to kaboom in a wrong time.

Also what is very expensive the component which nobody has needed 3 years ago at all, and now everyone needs this. There were no waiting drones at the time which can poke for days then get you from an unexpected bush. Where is the expence in countering that situation, maybe you need a kevlar umbrella? A Spiderman's net? A patroling drone over your head like in Alien Shooter?

Asymmetry.

They need wide deployment because an attack can come from multiple places.

And the attacker benefits more from decoys than the defender.

And the defensive system has to work against the most sophisticated threat but the attacker can use less sophisticated systems.

Or from first principles, warfare is expensive.