[SauronOfMordor]

Bombs are used against a fixed target, like a building. The hellfire missile is designed to kill a tank even if it is moving.

"Smart" bombs can be steered into their target. When you're dropping at terminal velocity, a small turn of the steering vanes can produce a very respectable lateral movement -- more than enough to stay on-target with a moving tank.

[Lokibob]

Hellfire missile uses a shaped charge, designed to penetrate armor.

Anything other than a direct hit with a 250 lb bomb would simply shake up the crew of a tank.

Different weapons, different missions...

Look, bombs do a lot of things, from putting holes in the ground (denying the enemy a runway, for example) to denying the enemy personnel land (cbu bomblets), to blowing up Palaces.