Lower cost and more
interesting sports like hang gliding and paragliding that have been
particularly strong in the USA.

If you want to fly something modestly attractive, paragliding is bot
cheaper than gliding at all. And not more interesting, either. Most
paragliders I know look with envy to the gliders. (Don't ask me why they
don't change, though.)

Another point is competition from power flying, which is a much more
attractive proposition in the US. I believe I'm correct in thinking that
flying power in most of Europe is much more expensive (and more highly
restricted) than in the US.

I don't know a single glider pilot who doesn't think that power flying
is just plain dull. Certainly useful, but nevertheless dull.