Ultimately Specialized has probably decided that the sales volume is not worth it. A new Epic HT will almost certainly have better trail and downhill capability as a Stumpy of even a few years ago, be lighter and have much better climbing. Its not that they would not sell any HT Stumpy's if they had them- its more they can move a vast majority potential HT Stumpy buyers to a HT Epic or a soft tail Stumpy. The lost sales not having it in the range would be very minimal.
Keeping hold of a HT Trail (the stumpy) - for which there is no longer big demand that cannot be met buy a salesman diverting to another model, does not make sense as the range of bikes is probably too large already, with too much overlap.
The Stumpy goes back to early 1990s (or is it earlier) - and has undergone numerous transitions. Its just a name - the bikes you like are nothing like the ones in the shop today.