He was popularized BEFORE he was champion because he was originally a young chess prodigy and, because we were at the peak of the Cold War, the American people saw Bobby as their Knight fighting the evil Soviet Empire and their Grandmasters who allegedly cheated to keep non-Soviets out of the championship cycle.
Note that a century before Fischer, Paul Morphy was recognized as the undisputed greatest player in the world, though official Wold Championship matches didn't exist yet.
Look up.