Short of T-Mobile telling us why, we can guess it's because it's now a year old device, or with all the folks rolling back from the Note 7 to the Note 5, it may well just be out of stock with no sign of it being available again.
I asked them online about it and they said out of stock. Weird though to remove it altogether like that. I called around and found one at a local store and got it last night.
I know t-mobile stores in my area are/were running low on Note 5s. Most had one or two 32gb models available, took me half a day to locate a dark blue/black 64 variant. I think with the recall their stock is all but depleted. Best of luck finding one.
Is Samsung still making them? If not, they'd have no choice but to drop them when stock runs out. I hope Samsung takes note and makes enough 8s for all of us who want one.