Likely because to do so would have been to endanger operatives and/or compromise strategy. A sane America would have accepted the pre-war intelligence that there were trucks laden with who-knows-what headed to Syria, but the braying by Old Hippie America and its acolytes was so loud, and so constant, as to obfuscate anything but their wailing, amplified by the complicit mainstream media.
Seldom, if ever, in wartime has an Administration had to deal with such a traitorous segement of society, such a traitorous press and a such a traitorous Congress baying, "The President's a liar! There are no weapons! Waaaaaaah!"
Under the cold light of history, it may well be remembered that America's resolve in the 21st Century War on Terror was weakened to allow nothing more than a nostalgia trip for a bunch of long-in-the-tooth 20th Century hippies looking for one last "revolution" before they finally kicked the bucket.
As it was in the '60s and '70s, so it is today: These people are idiots, and if the pro-socialist press wasn't enabling them, they would have long ago been laughed out of existence.
That's because other wartime administrations were competent.