Because most UK politicians (of both remain and leave) only had a superficial knowledge of the commercial, economic and political intricacies of the EU relationship. Brexit has shown that many UK politicians of all parties weren't qualified for their jobs and hadn't taken the time to learn what they should have known. Also that they had been happy for the media blaming the EU for decades, because it removed their responsibility.
If they aren't qualified, we the general public sure weren't to be given a referendum in the first place.

