I think a lot of it has to do with the fact pontoons are replacing the family runabouts in many homes. When I bought my first 'toon back in the '70's, there were just a handful of them on the entire lake. Now, you see them at nearly every dock. Marinas are at least 90% 'toons.
You have to ask yourself what happened to water skiing? Toons are just terrible at pulling when compared to anything even half way decent in a conventional boat. Yes, I know they can pull, but they'll never be in the same class as a ski boat. Point being, water skiing activity is WAY off. If you remove that interest from boating, THAT'S when the pontoon's abilities really start to shine. At least that's the case until you start talking about big water (all bets off at that point!).