Disabled also has one more advantage - it (and the 16/17 Saver) are the only two Railcards valid with no time restrictions. Senior has morning peak restrictions in the South East.
It's also available for some very minor disabilities, such as if you need a hearing aid in one ear (which is how my Dad has one, he'd not consider himself disabled). I'd rather have my hearing than a Railcard, but equally I'd rather have his lack of hearing in one ear than my post-thrombotic syndrome, which doesn't even qualify!