Actually, the P-39 wasn’t really used for killing tanks. “This is because the Soviet term for the mission of the P-39, prikrytiye sukhoputnykh voysk (coverage of ground forces) is commonly translated ground support, which is often taken to mean close air support. In Soviet usage, it has a broader meaning. Soviet-operated P-39s did make strafing attacks, but it was “never a primary mission or strong suit for this aircraft”.”
This is from the wikipedia article on the plane.
The heavy underbody armor on the P39 made it the predecessor to the A10 Wart Hog because it could take a lot of AA and MG hits without crashing.
Take a look at the “Black Widows” of the Russian Air Force and see what is mentioned about what they flew and how they liked it or not.
Also, the P-39 was easy to manufacture en mass so that is why the Russians took it over the P-40 Warhawk.