Bombing is somewhat insecure incineration. Perhaps the risk analysis was; we can leave them but there's a high chance they will be used against us or we can try to get access to destroy them safely but by the time that's agreed (if ever) they will have been removed elsewhere.
Not privy to those decisions myself, or to the nature of the 'weapons' being destroyed. Rather a lot of fuss was made about the USA leaving viable weapons behind in Afghanistan.
Very early days after Assad's abandonment of Syria, in a notoriously unstable region who knows what will stabilise it. Assad & Hussein & the Ayatollahs create stability of sorts, but at what cost?