I'm not sure that it's gotten more expensive, relative to other sports, cost of living, CPI etc. Like any interest - be it cycling, guitars, photography etc -, you can fork out minimal bugs bunny for entry level gear, or you can mortgage your house and go high-end. A lot of people like having expensive toys so they're prepared to throw down their hard-earned; my suspicion is that this is often a status symbol move, as said toys may more often than not spend more time gathering dust in the garage than being used for their intended purpose. Anyway, I digress...
Agree 100% with
@EsPeGe regarding the prices charged by certain marques such as Specialized, whereby an absurdly expensive bike doesn;t necessarily equate to a well-specced one. Then we move away from the bikes themselves; I've got mates with so many different helmets, gloves, shoes, riding gear etc that it's ridiculous and they can't. possibly use it all. So I guess the question begs, is mountain biking expensive, or do enthusiasts actually MAKE it expensive?