As John says, lathe is only a small part of the market. I can't remember if it was Art or Brian, but I'm sure their upper estimate was 10% of Mach users used turn, but more likely near 5%. And Mach is probably the most popular option for Turn applications.
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I'm not sure why threading is so hard. I suspect the problem is how more modern controllers have been programmed to handle general motion, that when they get around to adding threading, it involves major changes, so it gets glued on as an after thought.
I'd guess the old DOS systems were a pretty basic system. Move x steps each time there's a pulse from the multislot sensor, adjust timing slightly if too fast/slow, and you have a thread. The benefit that style of controller had, was the hardware and software was the responsibility of one entity, and you didn't have to program in lots of different options, along with all the other usually pointless functionality that users think they must have.