What I was trying to say I guess is that of course 61 keys were a sort of standard for many synths and I'm the last person who would say "no" to an additional octave on the OB-6 or the Prophet-6. The point is, the standard might have as well been different all these years, like the 49 keys you'll find on the Oberheim 4 Voice and 8 Voice. For a pianist, both is a compromise anyway. So you might as well ask why not every manufacturer chooses to add a properly weighted 88 keys keyboard - okay, that would seriously affect the production costs.
It has been mentioned in the context of the Prophet-6 release already that they were looking for a compromise between functionality and portability (Roland did the same with the JD-XA). You rarely see the keyboard towers anymore that have been around in the 1970s and 1980s. When people play live (the Prophet-6 and OB-6 are made to be performance synths in the first place), they prefer a compact setup. Compact can be one controller keyboard plus a laptop, or a few smaller, easy to pack devices for which you also don't necessarily need an army of roadies. Either way, I think that DSI/Sequential found a good compromise in the end. And that's precisely why I referred to the number of voices and the fact that the instument in question is monotimbral. If you had more voices and the option to split or stack patches, the actual keyboard size would be considered to be a much greater limitation than it is with this instrument's architecture.
Apart from all that it could also be speculated whether the 61 keys standard was not also a by-product of larger production parts in general. After all, the early electronic parts couldn't be made as small as they are today (not to mention the space that was sometimes needed to prevent synths from overheating), so the chassis would end up being larger anyway, not just for reasons of spacious interactivity.