I really like the Fujifilm X-Ex line in particular and use it more than my Canon 5Diii. (I mostly use the latter for action or if I need very tele or very wide angle lenses.) But the mirrorless lines are definitely sort of a niche. A lot of the potential customers are pretty locked into Canon or Nikon glass. Furthermore, for a lot of people, mirrorless doesn't fully replace DSLRs--though it increasingly can for many types of shooting. So you're left with expensive camera lines that aren't quite as versatile as a DSLR although they're significantly lighter and smaller.