Because that is how voting works. People choose and the most popular choice wins. Only they already voted, and it was to leave.
Why are you afraid to let people vote now then? They now have had more time to ponder how real were the promises and what the consequences are.
You may try to deny what´s happening, but the problem with reality is that is there even if you don´t like it: All the economic indicators of UK are going downhill, including the leadership in finances, the industrial capability and the ability of updating salaries with inflation even at full employment. The interest rates are also now going to catch the families on the wrong foot.
And no, you cannot replace more 25% of your exports without a huge damage to the country, that´s not how markets work.