Originally Posted by Chewy
Because Pym and Ultron define each other as characters, whereas Vision and Wonder Man... do not
Vision was actually defined heavily as being a Williams boy for his first hundred issues or so. In fact, until Wonder Man came back and they established that Vision was his own person, the official stance was that Vision WAS Wonder Man in a new body. This was central to many of the team's 1960s-1970s battles with the Grim Reaper.