[kyebosh]

Apparently "gay" & "retarded" are horribly offensive & insensitive descriptors, but "lame" is ok. Who knew?

[MiguelHudnandez]

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.