It also all depends on whether he feels comfortable enough with his size..
According to RadarOnline:
“He’s confident that by Christmas he’ll be in a good enough condition to be front and center of the group shots, but he’s refusing to commit to the shoot until the very last minute.”“Rob is demanding a staggering $250K to participate in the photo-shoot, claiming he’s the most marketable member of the family right now since he’s been out of the public eye for a long time now while he loses weight and cleans up his act.Every year the family get together for the iconic pictures- however last year the family didn’t do one which might be the case again this time round.
“He’s developed a major ego, which, combined with his sisters’ diva tantrums, has made this whole saga a total nightmare.”
Last year Kim admitted they had “ran out of time”, adding:
“I think this is the first year that we're not doing our card."