The anime has been running nearly non-stop since 1999. During that time the gap between episodes of the anime and chapters manga has grown shorter and shorter. This has lead to bad pacing, and heavy use of filler. New fans are faced with the daunting task of watching over 1000 episodes to catch up, meanwhile fans that want to re-watch the series have to wade through hours of irrelevant content to do so.
Higher quality animation
Post time skip One Piece has been knocked by audiences for the drop in animation quality compared to the what we see from the East Blue. Adapting the manga so quickly to meet the expectation of fans has lead to the incredible world and characters we love being unable to come to life with the level of dedication required.
The animation team has really been able to flex their chops in the Wano arc. While still a weekly release, they are adapting far less content allowing for a more artistic approach to the finished product. No new episodes of the anime gives the animators a chance to focus on a higher quality final outcome, as evidenced by the fight with Kaido, and the beginning portions of Egghead Island.