Words become offensive because of what they represent to the people offended by them. The R word used to be a perfectly acceptable clinical diagnosis, as did lunatic, Mongol, etc. "Handicapped" is an interesting one because it's still often used in the USA but would get some serious opprobrium in the UK. If you've ever been called something with the specific intention to cause offence, that word generally becomes offensive. If a whole community has a shared background like that, the word becomes taboo to everyone. Sadly for me, lots of people are OK with HoH and not OK with Hearing Impaired, which I prefer but can't say because it upsets other people! 
