Folks that have been around since it first aired on Toonami in the 2000's have stuck with it all these years and are committed to seeing it through. It also has the added benefit of not having a botched-up arrival like 4Piece being America's introduction to One Piece, and had a solid enough foundation to continue to pull followers in even with fillers. Bleach was similar in its following, except Bleach didn't play it safe and jumped the shark in canon too quickly. Naruto had very crappy moments as well, but was effective in reigning viewers in to have them look forward to what was going to happen next.
I guess Kishimoto has the benefit of the doubt that he actually ended the main storyline on a good note so those that know how it ended can look back fondly while those who are still anime-only are also hooked by knowing that it's coming closer and closer to coming to an end until the Boruto stuff starts happening.