One of the reasons I can think is that the people who make the show were lazy enough not to give the Team Rocket trio new Pokemon. The only other explanation I have for them not getting new Pokemon is that the show is mainly focused on Ash and Goh getting more Pokemon, battling, and them getting character development. Because of them mainly focusing on Ash and Goh, Team Rocket is falling behind in character development.
Also, Pokemon logic, as Da Viper said. That's all I have. I hope I've helped!