Because I've got an hard time when I've tried to use it back in the day, plus Mk-2 uses a custom thing anyway. I'm also pretty sure that some of the gfx modes aren't supported by the MC6847 core anyway, namely gfx mode 4 and its DIP-Switches selection.
Anyway if you or anybody else wants to try to hook it up for the plain PC-6001 go for it. I'm almost done with PC-6001 and sons for now, I'll just hook-up the MCU command list info, move around the joystick IRQ (that shouldn't be called at VBLANK but after the MCU command call 0x06) and remove the related PC-6001SR Vblank IRQ kludge ...
Speaking about the PC-6001SR ... I would mention the fact that there's an I/O bit that sets the HW to behave like the older PC-6001/Mk-2 models. The SR has almost everything changed from older models (bankswitch model, video emulation etc.) ... I really won't imagine right now how many things in the driver needs rewriting in order to support this crap