Putting the best players together at any level does not guarantee success, chemistry is also needed. So Major teams are allowed to maintain that chemistry season to season.
For teams playing restricted roster classifications, this is much more difficult because if you build that championship team and chemistry the reward is getting bumped if you keep it together. As a result, teams are faced with moving up or cut pieces and try again. All teams reach a level at which they are not comfortable moving up from as a whole. No different than getting an autobump at C worlds and half the team saying they don't want to play B, but they also can't play C again the next year with the exact same team.