There's a listing for the new frames on the SSK website. It says "dealers only" and "must buy five frames".
Price listed is $2125, but no explanation if that's per frame or for an order of five frames. (I hope it's for an order of five!!!)
Oh yes, and of course, they're out of stock!
But, there is a silly picture of a new one with a cheesy plastic AR style stock attached. Oh well....

And there's this statement:
"This receiver is compatible with Thompson Center/SSK G1/G2 Contender® barrels post TC Generation 1. The SSK-50 receiver is not compatible with TC Generation 1 barrels. Due to other barrel manufacture’s variation from TC dimensional specifications, we can not assure fitment to these manufacture’s barrels."

As someone who shot Contenders in the last century, when dinosaurs and IHMSA were alive and well, I have no idea what they're talking about. (If by Generation 1, they mean the cool ones with the cougar on the side, then I'm not interested in this new one.)
But, I do know this: When Warren Center was running the company, every barrel was guaranteed to fit every frame. If not, they went out of their way to make it right, no questions asked.
That company was run by nice folks!!

As for the OP question:
Why were Thompson Center single shots discontinued?
My "wise guy" answer is, "Because they don't take a 30 round magazine."