TLDR: Orbital bombardment isn't nearly the solution to most problems that we tend to make it out to be. It's good for softening resistance and knocking out key locations. It's not good for ensuring the complete destruction of your enemy. That's what ground assaults are for.
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Let's note though that short of completely destroying a planet with Cyclonic Torpedoes or the Life Eater, i.e Exterminatus, orbital bombardment is good for a couple things but not everything. It's great against big stationary targets, like cities and hive towers and big altars wot have a lot of skulls. Enormous armies that cover continents.
It's not good at precision. It's not good at a displaced, spread out foe. It's not even particularly good at cleansing planets. It's for knocking out strongholds, and large concentrations of the enemy where being off by half a kilometer isn't a big deal.
But when you need the charismatic, devious and dangerous leader of an enemy force dead, you need confirmation. Unless they're dumb enough to sit in the big tower while an Imperial fleet is up in orbit. That's why you send in Space Marines. To deal with the most dangerous foes, and to eradicate the resistance already broken by orbital bombardment.
I think as 40k fans we tend to conflate orbital bombardment with Exterminatus, and the fact video games let us call in pin point lance strikes with zero risk of incinerating our hero's faces. Planets are big. Like, rrrll big. It was one thing during the Great Crusades when you had an armada of 50 ships of the line. That might make short work of a planet in just a few days. But a Strike Cruiser and a couple frigates? That might take them months of systematic bombardment to wipe an enemy from the face of the planet, and that only accounts for maybe 40% of the enemies of the Imperium that even think like that. Orks don't care, they'll just live in caves and drag burning scrap down there to eventually build surface to orbit weapons. Tyranids don't care. They'll happily continue eating a planet alive while the Imperium tediously bombards hordes of lil critters. Chaos doesn't care. They know Imperial tactics, and they're probably doing some shit somewhere that when it comes to fruition, they win. Vanilla and Chocolate Eldar will just fuck off to the webway. Granola Eldar would just scatter to the 4 corners and wait for the Imperials to leave, or be dumb enough to land. Necrons will either retaliate with a green lazor or just bewp and teleport to another Necron Tomb.
I think as video gamers we're like "the most efficient solution is obviously to sit in orbit for however long it takes and bomb them with impunity until they give up or there's nothing that left can really oppose us." Not wanting to vaporize a productive world/planet/population, atmospheric shenanigans, warp magic, sun spots, stupid edicts, these are all good plot reasons why it often doesn't go like that. And then the sheer logistics of it all comes in to play.
But at the end of the day I think the best reason orbital bombardment isn't the solution to most problems is that it's lame. It's not heroic or interesting unless you're going for that stoic "tedium of real war" or "muwhahaha the power to destroy worlds" vibe.