This is probably because Russia needs ISIS in order to ‘justify’ their efforts to take over Syria and eventually the entire Middle East. They and the ChiComs, that is. They will probably save ISIS for last, or work out a deal with them once the others are done with. I suspect ISIS may actually be an invention of Russia. Because when it comes to Russia, and their KGB/FSB deception tactics, you really can’t put anything past them.
Assad needs his major bases, cities and towns secured before the Russians go after the sand people holed up in their eastern Syria towns with their slave markets and hills full of crucified bodies
Hopefully when the battle moves east Iraq will “ invite” the Russians to use air bases from which Americans have been disinvited since they were there to re supply the foreign insurgency they created
Russia will bomb, Iranian and Syrian army will go in and clean up
Then the focus will turn on the ISIS sand people if they haven’t already fled back to their foreign countries as “ refugees”