I mean, I just provided a potential reason.
They're 1 point out of their division lead (4 points out of the conference lead) despite getting the 3rd worst goaltending in the league this year. They may very well be thinking to themselves, "if we improve in net, we can make a bit of a run."
Talbot, while he's had some bad games, has had a .900+ SV% in 20 of 25 games this year. That's pretty good, especially in comparison to Quick, who's had just 9 .900 SV% games out of 24. Phoenix Copley, while having better numbers than Quick, has only ever played in 50 NHL games. Talbot has started 33 playoff games.
Grans, while a good prospect, is far down their depth chart and plays a position of strength for them. They may prefer trading him instead of a pick.
And yes, Demko would obviously be a much better acquisition, if he's available. But he's also going to cost WAY more than Helge Grans, who, depending on which list you look at, is their 5-10th best prospect. You're not getting Demko without giving up a good piece of your roster + picks + prospects.
Although sure, it might not be ideal in a 1:1. But like I said, maybe a Talbot (@50% retained) + Motte for Grans + a pick is something that's more doable. They improve in nets and add depth with playoff experience.