One reason could be that the UK voted no.
Another is that this Sarin lie was reasonably well known even back then.
This is because the Sarin ‘shells’ are like huge canisters, and have an effective range of only about two miles. I don’t recall the geographical details but at the time it was evident that they could only have been fired from rebel-held territory.
Huh, and I forgot the most important fact.
IIRC Kerry was maneouvered (by the Russians) into allowing peace in exchange for Syria giving up its chemical weapons. The casus belli disappeared.