If I'm remembering correctly, the photo shoot for the photos taken on the stairs was done sometime in late-October/early-to-mid November. By that time Alexandra Moltke was on her way out the door (and Vicki was about to be recast). Perhaps TPTB had already planned that Joel Crothers would be leaving. Roger Davis was definitely being written out soon thereafter. And David Henesy had yet to come back to begin the Quentin's Ghost storyline. But as to why Joan Bennett wasn't in the photo, I don't know - or why Humbert Astredo, Lara Parker and Robert Rodan were included because they were also soon to written out.
It may just have been a case that most of the missing actors simply weren't available that day...
As a bit of trivia, the first time a version of the '68 cast on the stairs photo was circulated was as the back cover of the official DS Vampire Club brochure that fans received when they wrote fan letters to the show. I remember that practically minutes after a girl in my 7th grade class brought her copy in to show everyone, almost the entire class was writing fan letters so that we could get our own copies and to buy the various packages the club was offering.
I wonder whatever happened to Lori Levesque? It would be wild if I ever came across her in fandom one day...