Why aren't Seventh-day Adventists able to eat seafood?

Adventists don't eat some seafood because it is unclean, according to Leviticus 11. However, if a fish has fins and scales, Adventists can eat it since this is allowed in The Bible.

However, since many Adventists are vegetarian they abstain from all seafood besides seaweed. Emerging science has revealed staggering amounts of Mercury in fish of all types, strengthening the Adventists' reason to abstain from all flesh food.