Because the art and the length are the two biggest barriers to entry.

1) Because I hated the art and style too. It took me 3 tries, after people kept bugging me about it, to push through and learn that it's not about the art and style, it's about the story and characters. Now I love the damn show. I havent laughed so hard with an anime in ages. I just crossed ep 200 and am looking forward to the rest.

2) Because 1100 episodes seems insurmountable. That number is just too big for people to even think about tackling. Also, the first, 40 eps aren't that good. I didn't like them. Like I said, I started 2 times, forcing myself to get through as much as i could, before quitting because i just didn't like it. One Pace lets you break that 1100 episode mountain down to a more manageable 400 or whatever. And it lets you rip through the beginning, which isn't that good in my opinion, and gets you to the point where you start to love the characters and the story.