Because you don't trade a franchise player for a package of middling players. Fabbri is a good player, but outside of him there's not much there. Isles have a deep prospect pool, picks aren't exactly what they would be looking for. Isles need good young players that could be top line players one day.
If it was a sign and trade the isles would get full value for JT, and at full value a young defenseman like Parayko has to be coming back.
You don't know that it would be a sign and trade and Tavares isn't getting you a Parayko player. If it does from another team, it will come with little else. It's been pretty clear that you need top end D to win the Stanley Cup right now. Parayko is on his way to becoming that and partially already is.
Islanders fans who think Tavares will bring back a huge haul in a sign and trade are right...to an extent. I don't think they are going to get those franchise changing players, ala Forsberg for Latta and Erat. The Islanders will get 1-2 good NHL players, 1-2 good to very good prospects and probably 2 picks, most likely first rounders in a sign and trade.
They need to rebuild their team and that starts with trading a guy like Tavares. I understand they have a good prospect pool, but can Tavares wait until he's in his mid thirties for those players to flourish and help? I don't think Fabbri is the logical choice for the Blues to trade in that deal, mainly because of his injury. Schwartz, Kyrou or Thompson, Walman or Dunn, and two first rounders is probably what the Blues would come with, if DA is smart and doesn't low ball Snow.
Schwartz is an outstanding player to teach younger players the right way to play. He's not elite, but he's a great player. Constant 50-60 point player. Kyrou has LOADS of potential to be a scoring forward. He can't do more in juniors and deserves to be on an NHL team (IMO). Walman or Dunn are both going to be good NHL defensemen. NYI need to reload and add to those already good prospects, and in two to three years will be ready to compete. IF Tavares wants to win while in his prime, he will not stay with the Islanders. I'm not saying he'll come to the Blues, but he will go somewhere where he can win, or at least have a consistent shot.