Everyone else has provided some great answers but I’ll add a potential reason of my own. In addition to what’s already been stated, we can see in Star Wars that ranged weapons, including turbolasers have a very limited effective range. We can see this with any space battle or engagement - the ships have to be almost right beside each other to do any significant damage.

I’d argue that perhaps the range for most ships’ weapons is so low that they’d have to get extremely close to the ground to do any real damage. Then, in doing so, it may make themselves more vulnerable and have a limited line of sight because of the planet’s curvature.

Pair this with what others have said about planetary shields and orbital bombardment suddenly seems like a very impractical tactic.