What has been the trend of MVPs’ positions in the NBA, beginning with 1955?

The reason why the trend shifted from centers to forwards and guards like other people mentioned, really is because of the 3 point line. The 3 point line had such an impact on the NBA I would compare it to the institution of the shot clock. You had the before shot clock era and after shot clock era. Now the division of center dominance and guard/forward dominance really is the before 3pt era and after 3pt era. The 3 point line was instituted started in the 1979–1980 NBA season.
Before the 1980 NBA season, out of 25 MVP awards, centers received 21 with the lone non center MVPs going to Bob Cousy, Oscar Robertson and Bob Pettit.
After the 1980 NBA season (after 3pt line introduced), the only centers that have received MVP are Moses Malone, Hakeem, Robinson, Shaq. That is, after 1980, out of 27 MVP awards, 22/27 went to forwards/guards (I don’t count Duncan as center here since he played PF in his MVP seasons). That is a complete reversal of the trend before the 3pt line was introduced. And it makes sense. if 3pters are more valuable and give the team more points, less reason to have a dominating big man who only makes 2 pters.