system
Given the following post, respond as if you're commenting on Reddit:
human
[Yu-Gi-Oh!] Why would Kaiba create new rules in Battle City that hurt his deck more than others?

Needing 2 tributes to summon each of his Blue-Eyes White Dragons is so much worse for him when he could just summon them normally before. Plus, if he didn't need 3 tributes to bring out Obelisk he'd instead be able to use them to activate its instant-win effect. Sure these rules apply to everyone, but there are few cards out there that can match Blue-Eyes or Obelisk in terms of power, so making it harder to bring them out indirectly benefits anyone he's dueling against.
gpt
Competitive balance and to prevent people from thinking the contest is rigged? If he's playing at a major disadvantage, then others can't think he's cheating.

Also, it may not be his decision to make. Remember, he's not the rights holder to Duel Monsters, that's Pegasus' company. If Pegasus thinks that he's holding the contest just to fuck as many people over as possible, he could issue a Cease And Desist order and yank the rights to Duel Monsters from Kaiba Corp's parks which no doubt get a lot of money for their holographic duelling tech.