why was anthony hopkins role uncredited

I'd hazard a guess that he was doing a favour for someone or fulfilling a personal bucket list item.

It was after all just a cameo; two fairly short scenes. But I thought he added that extra touch of gravitas to the movie.

And he had a couple of great lines...

"Mr. Hunt, this isn't mission difficult, it's mission impossible. "Difficult" should be a walk in the park for you."

Must have thought everyone would recognize him anyway and it would be an extra surprise.
I liked him as the boss. He wasn't a mole but ambiguous enough.Sth about his last scene was a bit chilling.

" You ain't running this place, Bert, WILLIAMS is!" Sgt Harris

Maybe this turned into some kind of gimmick, because in the original film, Emilio Estevez was uncredited as well. I don't know if anyone was uncredited in Parts 3 or 4.

Because he works for a secret organisation.