Because you need more than half the votes. If none of the Republicans and even 10% of Democrats vote against a bill, it will fail. The ACA is a long way from universal healthcare but even that compromise had Republicans try hundreds of times to repeal it.
Dems have tried many times, but it is always blocked.
https://en.wikipedia.org/wiki/Medicare_for_All_Act
https://www.congress.gov/bill/118th-congress/senate-bill/1655
https://www.congress.gov/bill/119th-congress/house-bill/3069

















In the last 13 years there have been 2 years where the Republicans weren’t blocking all Democratic legislation and that was at the peak of COVID where passing the American Rescue Plan and other associated pandemic recovery acts took priority. Before that the only other time there was a free path to pass something was when the ACA passed.