I actually did medicine myself (briefly) before switching to a more sciencey degree. I think the PBL and early clinical exposure that seem to be the basis of most courses these days don't attract the kind of people who are interested in histo, and this is a large part of why so few people are choosing it as a specialty now.

Regarding how the career pathways differ, it's a lot harder to do it via a BSc. You basically have to become a Biomedical or Clinical Scientist, and then if you excel and have a forward-thinking department they'll put you forward to learn to dissect and then report. The people doing it now are very much the thin end of the wedge, but because there are so few histopathologists coming through they're going to have to start opening this route up more and more.