Most major natural history museums are philanthropic or public institutions. Most (certainly not all) aquariums are for profit businesses. That's an important difference.
I didn't say that people don't learn at aquariums. I just suggested that that's not the reason they're built.
Many aquariums in North America are non-profit institutions (minus the emerging chain of SeaLife aquariums popping up, which are for profit). The major aquariums like Monterey, Shedd, Georgia Aquarium, etc. are run with an education and conservation mission.