In the short term? Because a good healthcare reform bill takes a lot of time. Unless you're the Republicans and want to ram through a terrible bill with zero hearings, amendments, or oversight, it's a long process.
For reference, the ACA took 8 months to get through:
In the medium-term, it might not be doable at all because the Democrats don't have a strong enough majority in the Senate. They need EVERY Democrat in the Senate to support it, and that includes Republican-lite Senators like Joe Manchin, Angus King, and Kristin Sinema. If they flipped seats in places like Iowa, Montana, and South Carolina, it might have been more doable. Specifically, Manchin is EXTREMELY and vocally opposed to single payer healthcare....which is what the public option most likely leads to as a matter of market logic imo.
It's important to remember why the public option failed in 2009. It wasn't a Republican that killed it: it was a conservative Democrat-turned-Independent who killed it. Basically, a guy like Joe Manchin.
In the long-term....well, it's pretty much the same story as the medium-term. If Democrats win more Senate seats in 2022, maybe more ambitious reform becomes more viable.