As we know Mr Woodhouse has to be told a thing repeatedly till it settles in his mind and he begins to understand it well.

With Emma going around declaring her intentions of never marrying, and Mr Woodhouse hearing it so often that he has accepted it as what will be.

Now here is Emma trying to change this very long and well established fact.
It would take a long time for him to come out of the 'never marrying-Emma' to 'now wanting to marry-Emma' by constant repetition.