Because no deal is right wing enough for some if it doesn't end with black people in chains or on the street begging for change. What in the debt ceiling deal benefitted Dem constituents? I'll wait
You are ignoring the political context though. The Republican caucus is crazier than ever, this is not the Boehner or Ryan Caucus where Democrats were skeptical that the GOP would allow default. Biden's people (probably) rightfully assumed that the House would be willing to allow default.
Also, polling showed that regardless of what led to default/economic downturn, the public would blame the President. Americans do not pay attention to the news and will only notice when their pockets are hurt.
What leverage did Biden have to negotiate with a House Republican Caucus that is fundamentally anti-government and would be willing to let the country default?