Because the President & his board are responsible for decisions of club during their mandate, that include ensuring they are fiscally responsible (Obviously, they have not been) - even once they leave/resign, they remain on the hook for decisions they take that are not approved. Any board can negotiate a deal, including debt restructuring, but ultimately it would need to be approved by the socis.
I agree it's BS but had Bartomeu stayed to attend the assembly and plead his case that the restructuring of the debt with say Goldman Sachs or Super League, he'd have a platform to do so. Now, he won't get to be able to spin his BS. Moreover, in the interim of new elections, the current President will be Carles Tusquets along with a management committee will ensure continuity until new board.
Tusquets has a PhD in economics with 40 years in financial sector - I suspect he will assess the current damage along with the committee and provide a report ahead of the election. Dust will need to clear so there is an accurate picture, a lot of misinformation will exist in the meantime