They ran on it for years but they were only pretending to chase the bus
That's not quite right - they genuinely thought it was a winning tactic, and had polls that looked to confirm that. There's at least three major reasons that they were wrong (and note that some of these apply equally well to some other major issues):
First, most people thought that
Roe was far more permissive than it actually was. A huge number of people who supported getting rid of
Roe and instituting tighter restrictions wanted rules that were much
looser than
Roe allowed.
Second, there's been pretty substantial shifting over time that wasn't fully reflected in the polls for various reasons. When
Roe and
Casey originally went through, they were massively unpopular, and it is very easy to miss that.
Third, and perhaps most importantly, there's a fuzzy effect that happens in polls. A
portion of people answering the poll Really Care A Lot about the issue, but there's another portion that's a lot softer or more apathetic on it. There's no real way to reliably tell the difference between the two. This can come from unexamined thoughts, especially if you're convinced that nothing's going to be happening any time soon - the answer you give is genuine, but is one you formed long ago and never really thought about since. The other place that this can come from is what I call the "Yeah, Whatever" people - if an issue doesn't affect you and you've never considered it at all, it is all too common to form a knee-jerk answer on the spot.
These, of course, are all interrelated. A lot of Yeah Whatevers and Unexamined thought the rules were far looser than reality, and a lot of Unexamined formed their opinions many years ago. More importantly, the nature of this issue put a lot more Yeah Whatevers and Unexamined on the "pro-life" side than on the "pro-choice" one. Then
Dobbs came down and ultra-strict rules started coming down. Now the Yeah Whatevers are finding that the question is a lot more likely to affect them and theirs than once realized, so they consider it. The Unexamined have to finally actually look at their own opinions. In this case, a huge part of these groups did so and realized "This isn't what I wanted at all!". Which is how opinion polls that were as high as 60-40 pro-life shifted to as low as 30-70 the other way virtually overnight.