Certain models are discontinued simply because they weren't selling particularly well anymore.
Rest assured, you have an excellent quality model there. The M&P15T II was a high-end model built to the highest specifications of any model in the M&P15 Series of Rifles.
Unfortunately, many felt that it was too expensive, so it didn't sell too well over time and was eventually discontinued.
Smith & Wesson hasn't had much success with higher end models because their exists an overwhelming sentiment that M&P15s are entry level AR-15 Pattern Rifles, so that's become their niche.
When it comes to their higher end models, the overwhelming sentiment expressed by people online is that for the amount of money they cost, you're better off buying from another brand like Colt, FN, or some luxury brand.
On the bright side, it means that the market has a number of excellent quality ARs available at lower prices because they've been devalued.
I myself just recently ordered an M&P15X — another of S&W's ill-fated high end AR Pattern Rifles — for less than half of what it originally was priced.
Here's an archived copy of the original product page on S&W's website:
M&P; 15 | Smith & Wesson
You made a fantastic choice of a top quality, American-Made tactical rifle, manufactured using state-of-the-art equipment and some of the finest materials available, unlike a certain faction of invaders armed with cheap rifles made out of stamped sheet metal. Don't let anyone convince you otherwise.