The answer is not so simple.
In the wake of the Arab Spring that removed several dictators from Arab and North African countries (Tunis, Libya, Egypt), USA and other countries supported the rising against Al Assad in the hope of bringing up a democratic government (and, to be honest, the Al Assad family were never very liked by Western governments, neither). So, the decision for intervention was because there was a "window of opportunity" to effect a hopefully quick and relatively bloodless regime change due to the popular uprising.
In contrast with the above mentioned countries, Al Assad regime proved more resistant and difficult to beat. Additionally, and as it happened in Libya (but not in Tunis or Egypt), people fighting Al Assad were not able to form an united front and soon split into several groups; some of these groups later joined Daesh. The proximity of the unstable Iraq also helped Daesh to win influence in Syria.
The surge of Daesh did in fact help Al Assad, as it gave Western powers cold feet about the idea of further help to the rebels (or even a more direct involvement to end the civil war). It also gave Russia a pretext for a more open intervention in favor of Al Assad (while the pretext was targeting Daesh, the bombings were often directed to the "pro-Western" forces opposing Assad).
As I see it now the USA has three options1:
Pull their full force towards direct intervention, topple Assad and wipe out Daesh. Militarily doable, but risks worsening the already strained relationships with Russia (Assad's patron). Worse yet, it risks leaving the country at the hands of a government that does not have the supports needed to control the country (as happened in Iraq). Recently there have been reports of USA diplomatic personnel asking for this way.
Pull out completely and leave its former allies at Assad's "mercy". This is bad on two accounts:
Leaving your allies alone will be remembered. If the USA just disengages and its allies are crushed, it will have more trouble finding new allies in the future.
One of the usual charges against Western intervention in Middle East/Third World is their support of corrupt, autocratic and cruel regimes (cf. Saudi Arabia, Saddam Hussein's Irak before the Gulf War, Shah of Persia, Pinochet) as a way of getting stability and control in a country while getting from that country the resources it wants. Giving free reign to Assad just because he is fighting Daesh2 and ignoring he is a dictator would reinforce that perception.
Keep pressure and get to the point where USA allies and Al Assad get to sign a peace deal that ensures the safety of the allies and some face-saving for the USA (and hopefully some degree of democratization in Syria). This would be the most desirable alternative now, and there have been attempts at it. But now Assad's regime feels that they are about to win and negotiations fail or cease-fires are ignored.
Add to that that it is electoral year in the USA and that gives politicians little spare room to maneuver, so unless circumstances change dramatically there will be no change in the USA stance.
1 DISCLAIMER: As my record of lottery winnings can prove, I cannot see the future. The options listed there are, at best, an attempt to compile all what I have read from different POVs about the issue, but there must be other options left. Handle with care.
2 Make no mistake, Assad and Russia are not fighting Daesh. They are fighting anybody who opposes their control of Syria, it just happens that Daesh is one of those groups.
