It was taking so long for the manga to finish the Monster Association arc that there needed to be something major to keep people interested. Adapting the webcomic straight-up wouldn't have been exciting enough. They already introduced a whole ass multiverse, so there are plenty of directions for the story to take.
Not that any of them will be interesting, though. OPM is a novelty series that should have ended early on. It doesn't work as a long-running series because a deliberately OP protagonist is only interesting for so long.