Multiple reasons. First is Fan-service. People wanted to see Frieza come back, so they brought him back. Second reason is convenience. It made a better secondary plot to have Frieza ‘plotting’ behind the scenes. Third reason is, Buu is just unfair. He regenerates from literally EVERY injury unless his entire body is completely disintegrated, has unlimited stamina, and can change the shape of of own body to whatever he wants. And that’s not even including the other unfair things, like his ability to instantly learn any technique he sees, his Candy Beam, and his absorbtion ability. If the creators let Buu enter the Tournament, Universe 7 would’ve won without much trouble at all. Even Jiren wouldn’t be able to defeat Buu without killing him. After all, if Buu never tires, and can dodge any attack by reshaping his body, then it’d be next to impossible to ring him out or ko him. Plus, he could’ve absorbed other fighters and surpassed Jiren, or even absorbed Jiren himself. So having Buu in the ToP would’ve been a mistake, since they wouldn’t have nearly as much trouble winning with a fighter like Buu. Part of what makes the Universal Survival Arc so great, especially now, towards the end, is the huge amount of trouble they’re having with surviving through the tournament. At the beginning, the main problem they had was the fact that every Universe in the tournament was gunning for them since it was Universe 7’s fault(specifically Goku’s) that the tournament was even happening, and therefore it was Universe 7’s fault they were all at risk of erasure. As the tournament went on, that became less and less of a problem, and instead, the power of the remaining fighters became the main problem. Now Jiren is the main problem. With Buu, none of these things would’ve been much of a problem.