As much as I wanted Stamkos, he wasn't leaving TB and why would he. TB is a contender and he is their captain, what's can the Leafa offered him at that time beside HOPE.
JT is different as Islanders is not even close to being a contender, they are only good bc Bailey is having a career year, Barzal is good but still too soon to call him an elite player and they don't really have a pipeline of players like TB.
From a player standpoint, Leafs is way way more attractive than Islanders, something we can't say the same when Stamkos was a UFA between Leafs and TB.