Perhaps a generational (and maybe class?) thing as well; it and related terms were vicious and influential enough to have motivated The Spastics Society to change their name to Scope in the mid-nineties [1]; I guess growing awareness of disability discrimination and bigotry also contributed to it being recognized as beyond the pale and disappearance from casual conversation with the corollary effect of some people outside those demographics being ignorant of how awful those terms were.

[1] https://www.bbc.com/news/blogs-ouch-26788607