This one of the biggest myths in the hobby, and one that annoys me almost as much 'scale-less fish are allergic to salt' or 'Bettas don't like big tanks'.
In 1992 freshwater fish accounted for approximately 96% of the total volume of imported fish to the US and 80% of the total import value. A more recent survey in Florida found that cultured freshwater and collected saltwater species accounted for $70 million and $4 million respectively. The top 32 species were all freshwater fish and accounted for 58% of the total imported value of the fish. In the US, 11 million people keep fish, but only 18% of these are marine aquariums. In terms of conservation issues, 90% of the freshwater aquarium fish traded are captive bred, while virtually all marine aquarium fish and invertebrates are caught from the wild.
The problem with marine fishkeeping is that it is very, very expensive to do properly, whereas freshwater fishkeeping can be done far more inexpensively, especially when you consider things like goldfish. Increasingly, marine fishkeeping is suffering from an image problem: there is good evidence that in certain places (e.g. Philippines) and for certain species (e.g. mandarinfish and Banggai cardinals) over-collection has resulted in serious declines in natural populations. Even if you banned wild-caught freshwater fish outright, that would only affect 10% of the traded species and most casual aquarists would never even notice. If you banned wild-caught marines, the hobby would basically vanish.
What I'd submit is that to marine aquarists it seems that their side of the hobby is the "natural evolution" of things. But that's often because marine aquarists don't explore the freshwater hobby in any great depth, so they get bored more quickly. They don't, for example, tend to be aquarists who enjoy breeding fish, which in my opinion is one of the very best aspects of the freshwater side of the hobby.
Cheers, Neale
Sooner or later most (not all, but a great portion) hobbyists graduate to marine aquariums.