Because Putin’s goal isn’t to defeat ISIS. He would like to maintain the Assad government in western Syria to assure access to a friendly port in the Mediterranean. An active ISIS in Eastern Syria, pointed at Iraq destabilizes the government the US supports, which is fine with Putin.
The US was attacking pro-Assad “rebel” forces.
Russia was attacking anti-Assad ‘rebel’ forces.
The rebel forces pretty much have gone back and forth between ISIS and getting foreign help/equipment and other masters around the middle east.
Saudi wants Assad out. That’s the US game.
Iran wants Assad in. That’s the Russian game.
Iran is taking over eastern Iraq, so my guess is that Russia really does want ISIS gone, and the US down deep wants Assad gone. Obama is playing both ends against the middle.