Because the cat is out of the bag. When the election was called Corbyn was down 20-25 points in the polls and in 6 weeks reduced that to 3. The polls also showed very large numbers of the public having a more positive opinion of him than before as they saw him campaigning on a national stage and liked it. Give this election another few weeks and we may well have won it.
Then we also have an electorate that by now is likely fed up of elections and referendums.
Maintaining momentum during a lull isn't as easy as you make out.
Also do you seriously think the Tories are going to sit back and do nothing?
Consider this. The campaign was undoubtably the worst in British political modern history. A car crash with a leader who couldn't communicate. While Corbyn was handing out free tickets to university may was alienating core voters and then pretending she never did.
Yet and yet the Labour Party gained about 5 more seats than when Gordon brown conducted his dire campaign.
Corbyn will have to gain around 47% of the vote if the minor parties are not going to recover.
The Tories will appoint a new leader and they will make concessions to the manifesto and that will gain them momentum. No one can moan if they remain in power as ultimately the electorate delivered a result that wants a Tory led coalition focused on its policies.
Kevin Maguire on the politics show said he couldn't see how Corbyn would ever win and that some voters who never believed he'd win will change allegiance next time round.