Theresa May has declared in her manifesto for the Tory leadership and the job of PM that:
"Brexit means Brexit and we're going to make a success of it."
Unfortunately politicians are in a Catch-22 situation. As politicians in a democracy they must be seen to follow the will of the people when it is explicitly communicated to them. Thus they dare not go against a referendum.
However if the referendum can be shown to have been flawed or that there is a legal requirement for the debate tot be re-opened then that may changed.