I thought it was because of Joe Lieberman, a Democrat turned independent, from a state that tries to bill itself as the "Insurance Capital of the World" [2]
If it had been 39 Republicans, it would have been 2 Democrats.
Obama campaigned for Joe Lieberman afterwards as an independent against Ned Lamont, who had won the Democratic primary. Lieberman tanked the public option for the Democrats, and Obama was paying his debt.
Democrats would have paid the price at the polls for getting rid of the public option. Lieberman took the hit for the rest of them; he was the cheapest, because he already publicly held right-wing positions. If they had needed another vote, they would have had to find another Democrat to risk their political career to do it. Probably with a cushy lobbyist job at a health insurer after getting voted out.
