Not to negate any other peoples arguments on how 3rd parties treat Nintendo consoles but I have found a key point people often miss when asking _why_ a game does or does not get ported over to a system. That being storage space. Whether it be on the system itself or the physical medium, it can be one of the greatest key factors.
Why would Skyrim ever be on a system like 3DS or PSVita? Not just that it would suck, but there is simply not enough storage space.