Lots of reasons. manufacturing facilities get retooled, changes in technology to reduce prices, initial lack of demand, changes in demand by OEM's, anything could relegate a perfectly serviceable device to the scrapheap.
This happens all the time with tech. Some very sound tech gets shunted for flavor of the month because you have to have some new shiney to sell. Add in that all of these devices are meant to be disposable and it changes constantly. We really aren't meant to repair them. I had a mouse I loved and the button broke. No parts to repair and not really designed to opened without destroying it.
The fact that the one I'm using has lasted fifteen years so far says that it fails the "disposable" test, and therefore doesn't fit well with that business model.
Which is, if I'm being paranoid, part of why it's not being produced.
Of course, most of my paranoid thoughts about why CoX got closed are looking to be pretty far off-base now, so what do I know?
Sometimes, business is just business, and the consumers rarely have all the relevant information.
Until someone like Ironwolf starts digging.
On that note - Ironwolf, once this is all settled, you wanna go after Microsoft and see whether they're ready to sell the rights to my trackball to another company?