I have got the stupidest answer ever!
Technically,the freezing thing is caused by kyurem.If kyurem kept on freezing every place,then it would not be able to produce more of the freezing thing.It might probably die.Kyurem generates a powerful, freezing energy inside itself, but its body will become frozen when the energy of the freezing thing leaks out.So,team plasma might have known that if it keeps on freezing every place in unova,then team plasma would not be able to take over this world since kyurem will be frozen if it produces all its energy.Then their plans will be ruined.So,probably after obtaining the DNA splicers,they might be able to fuse kyurem with zekrom and reshiram and it will be unstoppable since its BST increases to 700 from 660.So thats why they might have frozen oplecuid city as a warning for obtaining the DNA splicers.Otherwise,if they froze the whole unova,kyurem might probably be frozen.
Its just a Theory.
Hope this helps!(I guess).
P.S.-Dont ask unanswerable questions!