I think the reason this happens is because no matter what people say, the idea that if you can't finish a story within a sprint it will simply slide into the next one is anathema.

People spend more time on estimation when the consequences of mistakes are higher. If you can't realize that a story is larger than you thought and reprioritize mid-sprint without it being equivalent to missing a deadline in your old model, it's not going to work. In my experience this is the key failure mode of scrum.