Backblaze is operating at their own economy of scale with dedicated deals with suppliers, custom bare bones hardware, optimized processes, etc. It's always more expensive starting out as a little company for raw hardware and processes until you're big enough and mature enough to get the deals and processes in place.

That is also not the only service that Backblaze offers and wasn't their first. It could be that B2 is simply a way for them to offset their cost for extra capacity and are running it effectively near-cost for them.