Morality has to be flexible, partly becasue it must change to accept new ideas. For instance, homosexuality was considered widely immoral in the past, now most popel accept as morally ok, even if they dont approve of it themselves. This is an example of how flexible morality can save many poeple a lot of pain.

And, there is no one source everyone can agree on to establish an absolute morality, so therefore, any absolute moral code has to be imposed on everyone, making the whole system corrupt.