I think that there are some groups of people who prefer person-first language. For example, "person with epilepsy" is generally preferred to "epilectic person" (n.b. I do not have epilepsy). I also just looked into the history of person-first language and apparently it first arose in the context of people with AIDS, who were sick of being referred to as "AIDS victims" or similar.

In that light, I can understand why some people prefer person-first language. Myself, I am in accord with the general autistic community in calling myself autistic (as an adjective). Occasionally, amongst friends and kin, I may even call myself "an autistic".

There are others on this wider thread that capture some of my reasons why: I remember, shortly after I was diagnosed, I pondered whether I would take a cure for autism, if one existed. I concluded that I wouldn't — not because being autistic was a strictly positive thing for me (it certainly made my life harder in many respects), but because I didn't think that it would be possible to extricate the autism from what is intrinsically me — in short, any "cure" might as well be death.