I suggest it's because it's not commonly used today for its actual definition. People don't normally refer to themselves or people they love as "lame." Not so

I suggest it's because it's not commonly used today for its actual definition. People don't normally refer to themselves or people they love as "lame." Not so for "gay" or "retarded." A modern equivalent of "lame" might be "handicapped," even though it is not as specific.