Because it gives an encouragement for your opponents to vote tactically against your candidates. Tactically at this stage it's poor because an awful lot of Labour gains will be small majorities
Also is he really saying that if things go pear shaped in the run in (they won't), that he won't lead the party into a coalition / supply and confidence deal with another party? And that is the only question of this ilk that would actually matter. If he loses, he loses and yes he very likely resigns (because, he's not Corbyn), if he wins it's as irrelevant as him losing.