He chose not to defend the title -- "abdication" by the king, if you wish.

The public story is that it was because he found the format (120+30, + extra time + extremely well-prepared opponents, and every spice and wrinkle in every known opening well explored, so nothing exciting going on) not worth it. Instead, he wanted a mix of "fast classical" and much shorter time controls. (Informally, I suspect it was pissing contests in the FIDE org... chicken-racing e/o, them thinking Magnus couldn't possibly refuse to play.)

One case in point was his match against Fabiano from 2018: 12 straight draws in classical -- and you had to decide the classical chess champion in rapid tiebreaks anyway.