Roughly the same point made above by several posters with respect to surface accumulations of treifus which may or may not be easily cleaned. I have a feeling that there also may have been an issue as between the earlier microwaves which had interior plastic surfaces versus the steel and ceramic coatings used today (although plastic is still used a lot in the cheaper units..)
?The interior surface of the microwave over time gets greasy, such as a pot of greasy meat and so on, but such as our cooking, which is consists mainly of [boiling] water, [cooking potatoes or pancakes, with meat in it….etc……