It's more likely that he doesn't want to get drawn into a protracted ground campaign with no end in sight and do nothing but set up the conditions for another extremist organisation to rise following the American withdrawal - in other words, he doesn't want to do exactly what they did last time.
Exactly.
Also, the US and others are probably asking themselves, what's the point? If anything they do is going to be counteracted anyway, and/or the end result is going to be the same no matter how much political and financial risks they take, then why bother? Militarily, it's not worth it - especially when now we are seeing military intervention from Russia, Iran and others.
There is a humanitarian justification for getting involved militarily, but I would argue that it's a wee bit late for that - the bulk of the humanitarian relief for the Syrian people lies elsewhere, in the countries where millions are fleeing to, including the whole continent of Europe.
The question is, will Russian military intervention make the humanitarian situation in Syrian better or worse? That remains to be seen - but I'm guessing it will probably get alot worse before it gets any better, and that is also a reason why Western powers are reluctant to get involved. Sadly, no matter what happens, the civilian population of Syria face many years of strife because, plain and simply, the combatants on all sides (with a few exceptions) don't give a rat's ass about human suffering, and those who do are largely powerless to help. In these circumstances, the only option is to try to bring the entire conflict to an end, but that will not be easy no matter who is doing the bombing. The Russians have (for a variety of reasons, and I'm sure not all of them are good) stepped up to the plate and committed itself to a war of attrition. The US has, for once rather wisely, only committed itself to marginal support for a small number of groups, but the rise of ISIS has thrown a spanner in the works for anyone with the aim of ousting Assad.
Perhaps the best the outside world can do is to offer refuge to those who need it and assist Syria in the task of rebuilding once the fighting has stopped, if the place isn't a deserted wasteland by then.