It honestly seemed like bad politics for any of the Dems to vote no. We already knew where Manchin stood, but the other Dems had nothing to lose. Even if they really didn't want the $15 minimum wage, it didn't matter because the amendment was guaranteed to fail because it was never going to get Manchin, let alone the 60 votes it needed to pass. They could have voted yes and taken none of the heat for it failing.
Sinema just happened to catch the brunt of the backlash because she's been (rightly or wrongly) portrayed as another Manchin-esque problem child for the dems.