Korg are the only big 3 manufacturer to base their TOTL arranger on an actual TOTL WS. Hence, the ease of putting such a 'pro' connector on there (it's already available for the WS), although you would be surprised at how few 'pros' actually use the digital outs, due to the problem of clocking multiple digital devices together in a complex keyboard setup

Neither Roland, nor Yamaha have any connection whatsoever with their company's TOTL WS, except at the very core of the processor and some samples, so I/O is designed specifically for the target buyer...

Heck, if a 76 is too 'pro' to be included in an arranger, what chance is there of a digital out?