I think it is a few reasons
1. The main reason imo is the media is almost universally against it. It is extreme bias coverage on M4A. People are afraid of major change. When the ACA first passed it only had 35% support. The media did the similar fear mongering on the public option. The media also let republican politicians claim the ACA was a complete government takeover with giving the proper push back.
2. Some people do like their private insurance. People that have a good job, or that can afford the best insurance plans generally like the system the way it is. They do not think about the long term, like what would happen if they lose their job. They also don’t think about the people that have shytty insurance cover from their employer, or what happens if they have to have a procedure out of network.
3. A lot of people don’t trust the government. While I agree with not trusting government, I also don’t trust big corporations.
4. Because M4A is the most radical position that the left is pushing, there have been little to no negative coverage of a public option. However, if we stop pushing for M4A, watch how the Republicans, conservative Democrats, and the media flip on the public option like they did in 2009/2010.