Because he believes his regime represents us. LOL.
Rebels in Syria = Current and Future ISIS/Al Queda members
Just like in Afghanistan, even if they DID succeed in ousting the Russian-backed dictator, once in power, they'd simply point their weapons in our direction, so arming them is the height of insanity.
We need to simply cut our losses in Syria. All it will do is bleed money (and lives). So Russia gets their pipeline, big deal. In cementing Assad's hold, he'd be forced to deal with the ISIS presence, and we'd be off the hook for it.
While having ISIS around to keep the rest of the ME leaders lying awake in fear at night, has been convenient, it's time for it to end. We could easily destroy them (ISIS) in a matter of weeks, but we haven't because secretly, we like them there. (and if we did, some other group will simply emerge in the vacuum). We need to quit pussy footing it and simply go, hey, you want our help, we need to establish bases here...and those bases need to be US territory. Or, you can deal with them yourself when they come to saw your heads off....your choice. Bases are the ONLY solution for stopping this crap...and you can bet we know it.
Like Trump stated, when we get involved in these things, there needs to be a DEAL where we get something out of it, for the billions we spend, and the blood we spill. Simple as that really.
Let ISIS get too out of hand, and you know what's next....IRAN. If ISIS successfully started taking territory in Iran, we have the very real potential for a nuclear capable ISIS. Yeah...that's a fun one to ponder. As if Iran having them wasn't bad enough, ISIS with them would be a thousand times worse.