They do this to keep players playing, but they are doing it in a way that may backfire.
Some players may go "now that Fire Blast is inferior in every way, I need to grind for another Charizard with Overheat, maybe buying some incubators will help me get more Charmanders"
Some players go "wow, yet another unexplained, unannounced update that changes the battle parameters, fuck this shit because I am not going to invest any more time and money in this game"