In my neck of the woods, the number of baseball participants is rising, especially at the T-Ball level. That said, the number of players declines at each age level. My kids' league had 9 T-Ball teams last year but just 3 teams at the 13-15 age level.
I don't think it's that baseball is boring in general. But if you aren't any good, it can be VERY boring. Who wants to strike out three times and play RF two nights a week? Baseball is heartless. If you stink, it's painfully obvious that you stink. When kids realize that they stink, they quit.
If you stink at field sports like soccer, football or lacrosse, it's not as evident. It's easier to hide weak players and the kids don't get as discouraged/embarrassed as quickly