It's much easier to say that an intentional blockade of food is leading to starvation, and that is a clear war crime. Very simple argument, easier to prove.
Talking about a bombing campaign is more difficult when soldiers are mixed in with the civilians. We may be able to point at the situation and say "that's clearly fucked up", but courts don't work that way. They have to acknowledge that in a war, the army is allowed to destroy the combatants of the enemy. A certain amount of collateral damage in the form of innocent lives lost is allowed by international law. This makes it all much murkier and more difficult to prove what is or is not a genuine war crime. They can't wing it, or guess, or go by what it "looks like", they have to prove it, which again, is difficult.
Starvation and depriving food aid though, very easy to prove.