What was the reason? Seems odd to win the big one and not even defend your title.
Dickerson had won every other major title there was to win. The Olympia was the only one left.
He almost quit after losing to Columbu in 1981. But, Bill Pearl convinced him to give it one more try. Dickerson did so and won it all.
Once he did that, he had nothing left to accomplish, much like older football players who have great careers but are missing a Super Bowl ring. Once that get it (i.e. Jerome Bettis, Michael Strahan), they call it a day so they can go out on top.