He inherited and then backed up the policy of no chemical weapons use.
Having supported and backed up that policy with force once already, he can't just change his mind because the Paulite tinfoil hat brigade here wants him to do so, particularly in light of the growing Iranian threat.
The most foolish thing Trump has done recently is to have it be publicly known that he wants to withdraw from Syria.
That's what driving this. Having engaged, you don't get to call a 'timeout' and then cancel the game and walk home: the enemy always gets a vote.
It's never the wrong time for DJT to learn a lesson from the blood soaked, failed Neocon foreign policy of #41, #42, #43 and #44. Those of us who packed the halls for candidate Trump weren't chanting, "Kill Assad"! and "REGIME CHANGE NOW"! Instead we cheered when he exposed NAFTA, mocked the WTO, ridiculed the failed Bush presidency and Crooked Hillary. I'm still not convinced he's going to recklessly attack Syria and I hope I'm right. Just like the WMD lie, this gas attack is a sham, a lie pushed forward by Neocons and their globalist imps.