because it's the best in its category, by far ^^
and also, being quite old (and constantly good) gives it the advantage of being widely known and reknown, and thus sell well and on a wide scale :)
Finally the age span to read and enjoy it is almost limitless, and i would recommend it to a 8 year old as well as someone older than me, which only a handful of manga can achieve.
one piece just has so much imagination and creativity that it pulls one into its world. AN example of this was the skypeia arc which was something that showed off oda's creative flare. His imaginitive style reminds me of Miyzaki's (creator of Spirited Away, Princess Mononoke, My Friend Totoro), but Miyazaki is just on his own plateau when it comes to animation.
because i havnt found a better manga out that matches its potential
Hm maby the fact tha there is an endless list of good Characters many of which may stil show up again in the story
no i havent :)
Berserk is overrated. Simply put, it would be more well-known if it was more well-liked. Hence it is not more well-known than many other mangas. I do like Berserk. The Gryphon flashback arc is very, very good...the rest I feel is alright. However, on a whole I don't think Berserk is the best manga right now.
Berserk is great, but its audience is much more limited given its more serious approach, which explain why it sells less :)