It’s because early in Magic’s career he wasn’t LA’s best player and Kareem was still a viable MVP contender into the 80’s while the lakers were very much his team in the locker room too and voter fatigue for a lakers player likely played a role. In fact Kareem won the award in Magic’s rookie year and should’ve been mvp of the finals. The lakers really became Magic’s team around 1986 when he kicked off his first MVP season.
Boston on the other hand didn’t really have another player who was arguably the Celtics best player or leader. Bird from the start routinely led the Celtics in scoring by a decent to large margin while also leading the team in steals, frequently leading them in rebounds, and would regularly lead the team in assists or finish near the team lead.
His being a white guy in Boston and from the Midwest definitely got him some votes too though.