Likely scenario-
Nuke it and that 'leak' becomes a massive football field sized hole spewing out oil...Radioactive Oil at that.
And why isnt the idea taken seriously ?
Ok heres one idea. The Russians, when they nuked thier oil rig "gone haywire", it was a land based rig...far far away from people, and not connected to any source of water. So if it fucked up and didnt work ? No big deal. Just be quiet about it, fix it then bury it in cement!
The BP rig, if you fuck up, you fuck up...and congrats its now 10x worse, and spewing radioactive oil into the gulf aswell. The stakes are to high if it doesnt work. The Russians had nothing to lose.