The terms mental retardation and mentally retarded were invented in the middle of the 20th century to replace the previous set of terms, which were deemed to have become offensive. By the end of the 20th century, these terms themselves have come to be widely seen as disparaging and politically incorrect and in need of replacement.
In other words, any term used to describe something or someone in a negative way will eventually be considered disparaging.
Much like certain terms which describe members of various ethnic groups.