ISIS is executing Americans.
Assad is fighting ISIS.
We want ISIS to be defeated so they stop executing Americans.
Any effort Assad diverts to defeating the "good guys" weakens his ability.to defeat ISIS..
And gives it more time to execute Americans.
Our aiding the "good guys" would have increased his need to divert effort from defeating ISIS
Therefore it would have been "stupid" for Obama to have accepted the advice from McCain and others to aid the good guys. And Obama doesn't do stupid things.