Each card has a default DEC/Power value (based on card rarity), so power is determined by the cards you own or rent. The more cards you own or rent the higher your power will be, and you need a certain amount of power to advance to a higher league. Here is a pic that shows power requirements
https://i.imgur.com/KUNwaBc.jpg
The reason for this requirement is to create a balance for the reward system and to encourage players to invest into the game. Because as you move up in leagues the rewards you win become better and better, so they are trying to reward players who invest into the game more than players who invest less.