Because he’d make a dangerous player.
Right now, he’s got Iran over a barrel. He knows it. And he’s using the nuclear fuel as a bargaining chip, and he’s using it at just the right time.
Putin’s not stupid. He knows the true pathway to power is through economic, not military, strength, and he’s trying to keep pace with the industrious Chinese, and the best way to do that is to control the energy the Chinese get.
So Putin is doing what he knows best– get the Iranians “pot-committed”, where they’ve put so much stock in a nuclear program that they have to go “all in” now with the Russians or risk losing their investment and get stuck holding the bag. (Look at what the nuclear pursuit has done to the Iranian economy– to get nothing from essentially bankrupting their country will look very, very bad.)
Putin knows this, and he’s now putting the fuel squeeze on Ahmadinejad and his Ayatollah cronies. If the Iranians don’t agree to Putin’s terms, and I’m thinking a severe discount per bbl of oil shipped to Russia and a limit of fuel sold to China, then he’ll just withdraw the fuel the Iranians need to make their bombs.