Because doubts are now creeping in over the success of the attack, particularly the selection of the targets.
...So far, officials say, two factors make them wonder whether the facilities were still central to Syria’s program. At this point, there are no known casualties at the sites, which suggests that either no one was there during the evening, or they had been previously abandoned. And there are no reports of chemical agent leakage from the sites, despite attacks by more than 100 sea- and air-launched missiles.
Whether those particular sites were still in use or not, the conflict in Syria has demonstrated a larger truth: While it is easy to blow up Mr. Assad’s chemical facilities, it is also relatively simple for him to reconstitute them elsewhere, or just turn to a commercially available substance like chlorine to make a crude poison that any nation is allowed to possess.
That may explain why General McKenzie was a little more circumspect than his predecessors in forecasting the long-range effectiveness of the latest strikes.
“I would say there’s still a residual element of the Syrian program that’s out there,” he told reporters at the Pentagon on Saturday.
[source : A Hard Lesson in Syria: Assad Can Still Gas His Own People, New York Times, https://www.nytimes.com/2018/04/14/us/politics/syria-chemical-weapons-analysis.html?hp&action=click&pgtype=Homepage&clickSource=story-heading&module=b-lede-package-region®ion=top-news&WT.nav=top-news, 14th April 2018]
So that's handy, innit?
They first claim that Assad's chemical weapons apparatus has been destroyed (again).
But now they say that that may not be quite true, and that he may well be able to get it up and running again easily.
So next time there is another alleged chemical attack the righteous allies can point the finger at Assad again. And bomb Syria a bit harder again. Maybe even go after Assad himself.