Japanese tend to go to school or work 6 days a week. I wouldn't spend my lone free day off watching cartoons man. AND I LOVE ONE PIECE,but that's just me. Alot of ppl are introduced to a series because of the anime,but they usually end up buying the manga collection if they like what they see. Like I said, the rest is my OPINION based on things I've heard from fans in america and japan. They;re probably tons of reasons for it, but manga is more popular over there. I mean Oda-sensei makes a helluva lot more than the anime director of OP does. There's a reason for that.
As someone who has been "on the go" alot recently, I've always been able to find time inbetween classes, work, during train rides, etc to read manga. But it's been forever since I've been able to make time to watch anime on tv. I heard alot of Japanese are the same way, and had started using sites like Youtube to watch stuff they missed and dvd sales were slumping like crazy. Which is one reason for all those japanese law suits.