Answer: because of a quote Van Dyke made in a magazine article
Disney had seen a quote Dick Van Dyke had made in a magazine article. In the article Van Dyke he stated he did not like the way Hollywood was going towards dirty pictures. Remember, this is back in the mid sixties. This was a subject dear to Disney's heart, so he invited Van Dyke over for a visit, later giving him the part as Bert, the happy-go-lucky chimney sweep. As an interesting note, Dick Van Dyke so badly wanted to play the part of the doddering elder Dawes Sr. that he offered to do it for free. Van Dyke never appeared in "Camelot", but he did appear in "Bye Bye Birdie", the play and movie.