What's the reason / motivation to move up one league?

[Tactician]

One possible answer: to make users spend.

Each time you level up, the players in your squad lose 1 star, which means you will likely need to boost their quality by training, and/or buy new players. When a manager reaches the very high levels, things get more worst as it seems that only 3-star players are generated for the market; this might be a business ploy as users that reach those levels have a very high chance of being serious users, and hence they will spend money to get players from scout's list or the other expensive lists.