Senator's Manchin and Sinema are holding hostage the Democrats reconciliation bill, the central plank of Biden's "Build Back Better" agenda, that would address a range of important problems and is widely popular nationally to boot. Yet both Manchin and Sinema have been completely unforthcoming as to what they want to see. While they might claim that showing your hand is a poor negotiating tactic, they are not haggling over the prices of a used car. They are ironing out a policy agenda in which all Americans have a stake. So that's not going to wash.
I'm speculating that there's another reason. For Manchin it's his base and his donors; he's from coal country and doesn't want to upset workers (and owners) in the coal industry by supporting a move away from fossil fuels.
For Sinema, it doesn't seem to be her electors' interests that she's defending (Biden won Arizona while Trump won West Virginia, Manchin's state). It has been suggested, and this seems eminently plausible, that it's her donors, in particular the pharmaceutical industry, whose interests she is looking after. Admitting that would be electoral suicide, but she hasn't got a fall back explanation for her reluctance to sign on to "Build Back Better".