Western powers (or any powers in fact) do not just support a side in war because they like their ideology.
War is business. As a person or a party, showing public support for one side in a war can increase your political power, or mean political suicide. And if you bring in military support, you start bleeding money very quickly.
Assad has been vilified by western media (some say for good reason, but that's irrelevant in the context of this answer), and thus backing Assad would be an unpopular proposition. It would likely be political suicide.
If you look at it by nation, the European nations aren't eager to go to war. Going to war in order to aid an unpopular faction is very likely to end your political career in the next election. Earlier than that if you're unlucky.
Now look at the one Western nation that might join this war, the US. The president is suspected to have colluded with Russia to do some very bad stuff. If that president now spends a ton of US money to support a "villain" who's best known for being a pal of the Russian president, his career would implode.
What you can get instead of support for Assad is various statements of acceptance of continued Assad reign. That's something we've seen repeatedly before the Sarin incident. An example of what that did look like is this statement by the US ambassador to the UN from March 2017:
You pick and choose your battles and when we're looking at this, it's
about changing up priorities and our priority is no longer to sit
there and focus on getting Assad out...
Do we think he's a hindrance? Yes. Are we going to sit there and focus
on getting him out? No. What we are going to focus on is putting the
pressure in there so that we can start to make a change in Syria...
We can't necessarily focus on Assad the way that the previous
administration did...Our priority is to really look at how do we get
things done, who do we need to work with to really make a difference
for the people in Syria."
This is essentially as far a western leaders could go in supporting Assad without jeopardizing their career.