According to Sports Seoul News Kim Hyun Joong wasn't able to join his members' reunion because that time he has a shooting schedule for his variety show in SBS 'Barefoot Friends'.
Now that we know the reason, I hope some fans stops complaining and judging our Leader. He's really busy right now and as a solo artist, his individual activities are his priorities and I'm sure he'll make time for his members someday.
Patience, Triple S!