Unless the Jets knew he wouldn't agree to a contract with the new team prior to being traded, thus bringing down his trade value even further. That may have been the case actually -- considering it's been like a month since he was traded and still no long term extension with NY, it appears Trouba is in no hurry to agree to that long term extension.
I don't doubt he will probably end up signing long term, but if he hasn't signed with them in a month, odds are he wasn't going to agree to the extension prior to the trade happening - and the Jets likely knew that and didn't want other teams to know.
Another potential reason, if Trouba let the Jets know he only wanted to play for NY long term, it's likely the Jets didn't want other teams to know that. Why let the Penguins talk to Trouba and know he doesn't want to stay with them long term? Why let the Rangers hear from Trouba's mouth that he ONLY wants to play for them....that would decrease his trade value even further because now NY knows they are literally competing against no one else for his services.