Because it covers far more Americans than were previously covered, it prevents discriminating against people with pre-existing conditions, and provides a better safety net for people who need expensive medical procedures. If there's one thing that animates people across party lines its having access to healthcare so they or a close family member don't die or spend the rest of their lives avoiding medical bankruptcy.
This is precisely why single payer will eventually get adopted - it makes too much sense and health care is the number one issue that people vote on.
