Good question,
The big answer is...they don't. Burton currently has 12 men's models (not to mention women's and kids) in the $300-$400 range, and that is the bulk of the boards they sell. They have many more in the $400-600 range.
Also, Burton owns Forum, soooo, even more boards available in the lower price range.
Also, MSRP is not the same as price paid. Most of the highest priced boards (a rapidly shrinking market) sell for much less than sticker. These products are great for brand credibility, "We have a $1,500 board", more than actual sales.
But, it should be said that The oldest names in the sport:
Jake Burton, Tom Sims, Burt Lamar, Rob Morrow
deserve a ton of credit. Those guys and their brands took snowboarding from flex-less, edge-less, camber-less, sidecut-less planks, to very close to what we are still riding today. That all happened before a lot of the newer brands had ever produced a board.
Core snow-boarders remember and respect that, although some key-boarders (hehehe) do not. Along with that goes a little more cred and in some cases a slightly higher price.