Given that it will be 15 years between the release of Legacy and Ares, it's possible the filmmakers felt a standalone sequel over a direct sequel was warranted, for people who may not have seen either of the previous films. It's also entirely possible the characters WILL appear. Simply not as the main characters. Similar to how Kevin Flynn (Jeff Bridges) and TRON (Bruce Boxlightener) were the main characters of the original film, but were secondary, although still significant characters in Legacy.
It was announced that Jeff Bridges will return. So it's likely that others will as well.
