To start with, let's look at the obvious distinction between the parties' respective stances.
Trudeau's position has been to declare that he is "unequivocally opposed to any sort of coalition". In other words, if one believes Trudeau's public statements (and once again, we shouldn't), the Libs don't care what kind of terms they could agree to on policy, cabinet positions or any other area, but are opposed to cooperating with any other party as their lone inviolable principle. And if that means propping up the Harper Cons, that's a result Trudeau is happy to accept.
In response to that position, Mulcair is now saying that he's not going to talk any more about a coalition in the period leading up to the election:
On était prêt à mettre beaucoup d’eau dans notre vin, parce que notre priorité est de nous débarrasser de Stephen Harper. M. Trudeau a dit qu’il serait peut-être prêt à travailler avec le NPD, à condition que je n’en sois plus le chef. J’ai soulevé le projet à quatre reprises. À un moment, on se lasse. C’est fini ces histoires-là. On regarde l’électorat et on leur dit que s’ils veulent du vrai changement, ça passe par le NPD. C’est dorénavant le seul propos que l’on tiendra là-dessus.So the Libs have stated their bare refusal to even consider talking about any coalition, and have indeed stated that they'll never accept one in any shape or form. And in response, the NDP has said it won't bother continuing to make an offer that's been rejected.
Which isn't to say that I agree with that choice.
In fact, it should be clear that the period leading up to the election isn't the point where any deals would be reached in any event (since pre-election collusion along the lines of the Red Green pact has never been on the table). Instead, coalition discussions would happen only after the party standings are determined in October. And I'm at a loss as to why Mulcair is backing off any preparation for that discussion as the election approaches.
As I've pointed out before, any party presenting itself as focused on ending the Harper Cons' stay in power should be zeroing in on why that goal is important, rather than leaving any room for implication that working together isn't worth the effort. In the "change" versus "more of the same" argument that will ultimately determine whether the Cons cling to power, it doesn't serve the public interest to hint that there are circumstances where a party would turn down an opportunity for change.
Moreover, as has been clear for some time, a strong majority of voters - and particularly the ones likely to be choosing between the NDP and Libs as their options - prefer a coalition to the alternative. So it's hard to see why Mulcair would be backing off of his predecessor's strong stance that a narrow-minded focus on false majorities and resulting refusal to cooperate represents part of the broken Ottawa that the NDP is trying to fix. And indeed, a departure from that party-first mindset represents part of the change that voters are most likely to prefer even in choosing among their opposition alternatives.
In sum, Mulcair's stance is far from the outrage that the Libs' spin machine wants to present it to be, and in fact leaves open the possibility of a later coalition which Trudeau has repeatedly tried to shut down. But it does involve needlessly forfeiting what should be an important issue for the NDP and for the broader political scene alike.
[Edit: fixed typo, wording.]