McCartney adapted his style to fit according to each song. For example, his bass playing on his own ballad Here, There, and Everywhere is almost exclusively confined to root notes, because that's what worked best.
Here Comes The Sun isn't a ballad, but it has a number of tricky parts with highly unusual timing. Paul, I think, figured that there was enough going on in the song without adding, e.g., a bass countermelody.
His playing on George's ballad Something, on the other hand, while widely beloved by bassists who are frustrated guitarists, was arguably a rare misjudgment. Harrison, upon hearing the bass part that Paul had played, asked him to play a line as simple as the ones Paul invariably used on his own songs instead.
Paul refused to do so. Which I take to indicate that it was meant as a middle finger to George. Why? Who knows.
)
