Why was there no purely human council?

There was never supposed to be an entirely human council. It is true that Udina suggests that at the end of ME1 but a human-only council is never confirmed to have been a fact in subsequent games. If you let the old council die, the new one will be led by humans but won't be purely human. That's how the council is presented in ME2 no matter what Udina says at the end of ME1.

As for an explanation why exactly the Udina suggestion fails - none is given in the games. We can only speculate that such total control was simply not allowed by the other races in the galaxy.

You can't get a purely human council, but you can get one that is mostly human controlled.