It’s a good question and might depend on whether Native Instruments owns the license to it for their S-series controllers. It’s also an open question as to whether it might be a drop in replacement for the current Fatar keybeds. I’d love to mod my OB-6 if it is!
Yamaha just confirmed the new 88 key version of the new Montage will feature Polyphonic aftertouch.
No one knows whether it is a design choice. It could be for any reason.