I feel like a few reasons. There are snowboard companies that just make total crap boards. Kids that want to be cool and get a snowboard can spend $150 on a Morrow board, or another cheap brand, that is just a single shitty piece of wood, with shitty fiberglass, and shittier edges/topsheet. The people that ride it, love it. They go down the hill falling leaf style for two seasons, then realize they should get something better.
Also, to make a board it's once in the press, less edges/sides to clean up, bevel, etc.
Skis, two presses, twice the clean up work out of the press.
Also, a flex pattern and turn radius on a ski matters a little more than on a board I think. I've see boards that hardly have a flex "pattern". It's just one single flex throughout. skis are a little more unique. Also, for skis you can't get away with a bad core underfoot. Bindings will just pull out. Snowboards have the built in inserts, so that takes care of that problem.
and like someone said, skis range from $299 - $1000 or so. Any reputable snowboard brand makes boards in the same price range.
I don't count the brands that sell $100 snowboards as real boards or core brands.