I don't go around using that word because of how many people find it disrespectful. But, and I ask this out of honest curiousity, why is it offensive in the first place?

I see it as synonymous with 'idiot' or 'stupid' when used colloquially. The argument that it's a medical term doesn't really hold as 'idiot' and 'moron' are also medical terms that refer to a lacking of intellectual acuity. In many ways 'retarded' has the same meaning both colloquially and medically. To be mentally retarded is to be mentally slowed or lacking that similar mental acuity that 'idiot' or 'moron' convey.

Retarded just means slow and it's a perfectly apt description. Where I think people get confused is when retardation is linked with a specific attribute like physical retardation or emotional retardation, those convey very different meanings.

I'm not saying that we should start using it again, but that I find it odd how society has latched onto a very specific word and labelled it as bad in the matter of a decade. At the end of the day, any word that can be used to insult or demean, is rude. It's not the word being used, it's what is meant by them. The term 'Cis-gender ' is also being used in a highly exclusionary way and often times is conveyed as an insult. However, it's real meaning is not insulting in the least.