Let's see,
While Magic had a lot of talent around him, he was the undisputed "lead" player on that team.
Isiah Thomas was the "lead player" on his team. In fact, the two best players on that team were guards.
So in the last 24 years or so, the teams that won championship where the best player on the team was NOT a guard were: Spurs, Rockets, Celtics, and the Lakers (Shaq/Kobe).
The teams that won championships where the best player was a guard were: Lakers (Magic etc), Bulls, Pistons.
Its hardly a dominating historical pattern.