They had 59 votes from Democrats for the public option, not single payer. Public option is to let people choose either a non-profit government insurance plan or private insurers. They needed 60 votes to break a Republican filibuster. Joe Lieberman (Ind-CT) said he would filibuster the public option but would vote for the ACA without it.

Whoops, good correction, I did misremember public option versus single-payer, and I guess technically Liberman was not a Democrat at that point, but he had been for a while up until shortly before then and still had committee chair positions from the Democrat leadership, so it mostly feels like a distinction without a difference.

I still think this is an important historical detail that seems to go under the radar a lot; if anything, it gives me yet another reason to disdain the filibuster!