The problem is that Hellsing's mainly bought by twentysomethings who have $80-$100 to throw down per episode, while One Piece's target market is prepubescent and regularly have to pool their allowances to buy a $4 copy of Shonen Jump to share if they also want to get gum, soda, or Pokemon cards that week.
It's also a really bad way to keep your brand out in front of the mass market unless, like Hellsing Ultimate, the series is over but there's fan demand for a recap.
88% of one piece fans are over the age of 18 with 32% being 30-49yrs old.
the problem is a lot of one piece fans are normal people and aren't crazy enough to pay $100+ per episode.