I also go with the fear & ignorance theory - losing control is a basic human fear & people have a tendency to dive straight into a paranoid self preservation mode when they encounter something they can't (or don't wish to) understand. There's also the element of MI still being an easy target that is less likely to attract criticism for the one(s) doing the targeting.

Rather like various groups in history have had to essentially self organize to highlight & protect their situations, I've a feeling that those affected by MI will slowly have to go down a similar road, in order to get a 'voice'. The advocacy of groups like NAMI is a start, but we're a long way from the time where early recognition/preventative health efforts for mental wellness are given the same reception/resources as those for more obviously physical conditions already receive.