Nobody's happy with it but leaving it alone to fail was even more untenable. Basically the Dems wouldn't touch it once it passed to fix its problems, and refrained from enforcing its penalty taxes because of the blowback that would happen, but those penalties and their forced participation rate were all that made the financial model for Obozocare work at all.
For structural reasons in the Senate, straight repeal couldn't pass because even if all the GOP Senators agreed, they still wouldn't be able to invoke cloture because zero Dems would vote for it, leaving a budgetary bill and reconciliation as the only viable approach to get a complete bill out of the whole mess. There is literally nothing that can pass the House and get 60 votes in the Senate as a repeal and replace package, it pretty much has to be fixing it in the budget bill process or nothing. And 'Nothing' comes home to roost in the mid-cycle elections, because most of the electorate expects whoever controls the House and Senate to fix shit that breaks while they're in office, no matter who broke it.
It was a damned if you do, damned if you don't situation. There are nowhere near enough votes to come remotely close to passing something that would make laissez-faire Capitalist Conservative purists happy. The moderates in the GOP don't own enough seats to get anything to win a House vote, without either bringing enough Conservative purists or moderate Dems on board, and neither of them want to play for different but related reasons. Letting it fail completely also results in a lot of GOP seats going bye-bye in 2018.
At this point it's best to get some kind of a partial fix on the most FUBAR parts of Obozocare because the alternative is taking the blame for doing nothing while Rome burns. Even doing that enrages all the purists on the Right while the Left is screaming bloody murder about how Trump is killing everybody in America. The best of a lot of bad options is to move it as rapidly as possible and hope the shit calms down in a year, when the mid-term campaigns start kicking off.