It's easy to commit genocide by bombarding Earth from orbit, but you really don't want to.
1) Once you've finished destroying the birthplace of humanity, all of your shiny destroyers will be gone (destroyed by Durgas). Remember the UEF can shove Durgas down your destroyers' throats before they can jump out. Even if you somehow manage to save your destroyers, your support craft and fighter squadrons will be in tatters.
Bombers are slow and ponderous. It takes just a minute or two for a destroyers jump drive to re-charge.
That, plus it's escorting fighter complement = bye, bye Durgas.
Go ahead...do a little sim and you'll see what I mean. By the time bombers launch, evade the interceptors, get a lock, fire, and the SLOW bombs traverse the distance or hit (assuming they're not shot down) - FAR more than a minute.
But time enough for a few beam salvos.
2) GTVA populace probably wouldn't take too kindly to their government committing mass genocide (probably leading to huge civil unrest and rebellion - You could always crush this with more orbital bombardment, but really now?).
You dont have to. With Earth as your mean shield, the UEF will be responsible for any damage.
And the threat of destruiction might be enough.
Note that it's not even necessary to do even that. A massed attacked on the UEF fleet is all that's necessary. Without jump limitations, logistics problems from bringing in an entire fleet are non-existent.
4) You've achieved nothing politically. You've now shown everyone involved without a doubt that the Tevs are officially the bad guys. Remember that the whole damn point of the war was to make sure we were able to fight the Shivans by getting rid of Ubuntu pacifism. Not only have you needlessly wasted a lot of our capital ship deterrant, but also probably galvanized Ubuntu beliefs and splintered Terran population (see point 2). The Zods probably wouldn't take to kindly to genocide either.
Given how tight the media control by hte GTVA is...who would know? And who would be blamed? Heck, with carefull media manipulation, yo ucould even blame the UEF for any damage caused during hte battle.