I haven't looked into any reasons past the only one that actually matters, which is that people who actually have an intellectual disability prefer it.
> which is that people who actually have an intellectual disability prefer it
I think it's reasonable to be skeptical of consensus on a group of people on the matter of what they like to be called. Consider, for example, the backlash on the term "Latinx" and, more recently, even among some of our Black neighbors over being included in BIPOC.
I'm not trying to get into a whole thing here, it just seems to me that "intellectually disabled", taken at literal value, is more disparaging (and more open to slur-ish interpretation) than "mentally retarded".
Your links don't appear to support your position. None of them say that the people in question prefer this appellation. The WaPo piece expressly prefers "people with intellectual disabilities" (with Eades emphasizing that the inclusion of the word 'people' first is that makes the phrase better), so presumably he'd have a similar objection to the term "intellectually disabled."
The NIH doc doesn't even include the phrase "intellectual disabled" but does suggest the phrase "mental retardation" (see term number 13), so I can't imagine why you've included it if your point is to suggest that the former is more humane than the latter per this source.
In any case, if indeed there is the kind of consensus you assert, I'm happy to oblige it. But I think it's reasonable to wonder whether that's so.
To be transparent, I looked up what the preferred term seemed to be because I didn't know offhand and I wanted to offer actionable advice. Most resources seem to recommend "person with intellectual disabilities" ("intellectually disabled" being the adjective form of that, although I could be mistaken). The NIH doc recommends “learning disability” specifically to be used in place of the r-word.
So yeah, I'm also happy to use a better term if you know of one. I’m not really trying to prove a point here, per se — I just know there is widespread consensus that the r-word is a slur and I’m trying to gently help us move past it :)
Edit: why the downvotes? I'm pointing this out in good faith.