> When I think about it, I can't think of any compelling reason why that should be the case. Trapped electrons vs magnetic regions; I have no intuition which one of them is likely to be more stable.

My layman intuition (which could be totally wrong) is that trapped electrons have a natural tendency to escape due to pure thermal jitter. Whereas magnetic materials tend to stick together, so there's at least that. Don't how much of this matches the actual electron physics/technology though...