They're trying to allow players to have more combinations when it comes to strategy,so by adding evolutions to many of them they get stronge, that's what I think
It's to allow a wider variety of strategies, and it also helps from people resorting to choosing the same pokemon because those few are placed in higher tiers.
It's my job to help the next generation, and set a good example for them. And for that I'll gladly lay down my life.