To answer the question posed in the title, it's because One Piece is unfathomably good. With a staggering 490 million copies sold, One Piece recently usurped Batman as the second-highest selling comic of all time, and it doesn't seem like the series' upwards ascent is going to stop anytime soon. Now, maybe you're like me and were initially put off by the goofy art style, or perhaps the 1000 plus chapter commitment required to interface with One Piece throws you off. I can understand your apprehension, trust me. However, on the other side of your comfort zone lies one of the greatest stories of all time, but don't just take my word for it.
In the video linked above, Super Eye Patch Wolf, dedicated anime YouTuber extraordinaire, explains why a series with intentionally goofy art and an increasingly steep number of volumes is well on its way to becoming the greatest selling comic in history.