I have come across this issue several times in the past couple of years by the virtue of reading comments under articles. These criticisms are a part of the public record.
> And why do you suppose that is?
Most people learn words by figuring out what others call a certain thing, and then reusing that term to build a shared understanding of the world. This word is no different. At no point did this label acquire (in my perception) any connection to ability (mental or otherwise) until I first heard of it from US-centric English speakers. In my perception, it's the opposite: anything that is "retarded" is perfectly capable of being better designed/implemented or of making better choices, but instead actively chooses to do the worst possible thing for seemingly irrational reasons.