It is much easier to vote no on everything put forward by Republicans than spend political capital on reforms one's backers don't support.
Manchin has nothing to lose because he’ll more likely than not retire and Sinema couldn’t get away with being the leading voice/deciding vote of opposition to major Democratic policy proposals in a left-trending Biden state in which there’s no shortage of credible primary challengers. She could pull that stunt/put on a show in a 51D-49R Senate, but certainly not a 50R-50D Senate.
What reforms contrary to the backers' interests are we talking about here, and who are the backers in this case? I don’t think ultra-wealthy "backers" of Sinema are all that different from or more opposed to expanding the court or nuking the filibuster (to give just one example) than Schumer's backers.