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