I think Biden ed up by reneging on his pledge to veto the Rob Portman infrastructure plan if it wasn't passed jointly with BBB. Once he backpedaled on that and basically said he'll pass whatever congress sends him first I think it emboldened Manchin/Sinema into thinking they could bully him.
Sinema is just acting in bad faith. Manchin is acting out of self-interest and he's also just a bumbling idiot who's obsessed with 1990s era ideas about "work requirements" and "means testing." At one point he demanded that paid family leave have a work requirement
The party committees should create a new policy that they won't support any candidate who takes money from No Labels PAC and/or doesn't support obvious like giving Medicare the ability to negotiate drug prices.