I'm pretty sure I read somewhere in one of the lore posts that ground combat isn't feasible for some reason. Actually, I just remembered why- it's just much easier to blockade the planet and intercept anything coming or going.
There aren't many (any?) colonies in the sector that are self-sufficient, so if your goal is the colony, you can usually force them to submit by intercepting inbound transports and starving them of whatever resources they lack. If your goal is to take possession of their exports, it's cheaper to pirate the goods from outbound transports than to commit yourself to an expensive invasion and all the expenses of producing the goods yourself. Besides, in any violent invasion, there's bound to be some collateral damage. It's easier to replace a product than the infrastructure that produces it, and blueprints are irreplaceable.
Personally, it would be a fun feature to add since it would add some depth and another toy for modders, but it's probably not worth the development time. It's probably really down low on the priority list, right around where you'll find 'multiplayer.'
EDIT: Oh, and if your goal is to destroy a target, it would be easier to either bombard the target from orbit or just redirect an asteroid to their general vicinity.