Except that Labour did not campaign on stopping Brexit; quite the opposite. Corbyn (who, depending on who you ask, is possibly a bigger Euroskeptic than David Cameron ever was) pointedly did not position Labour's election stance as being anti-Brexit. That was up to the LibDems who, in a FPTP system, were not going to win anyway largely because everyone still hates them over the college fees reversal.
Corbyn's Labour party always insisted that it would negotiate the "best possible Brexit," one that was more or less a "soft Brexit" but, just like the Tories, still a Brexit with all that entails. People were pleading with Corbyn to do exactly as you described and make the GE a mini-referendum on Brexit by Labour campaigning to reject Brexit...but Corbyn didn't.