Plenty of reasons.
1) UR wants to build suspense.
or 2) Those characters are cowards.
or 3) Those characters have their own opinions on whether or not they want to join a clan (How do you join a clan anyways? Do you get jumped in, sign a contract, tell them you wanna join and they just let you, get an invite? Or does each clan have their own thing? )
It could also be 4) Those characters were just arbitrarily mentioned, but UR never intends to release them.
I guess anyone can join if they fit some criteria.. Like you're a Frozn if you live in the mountains, Uppers if you're rich, Montana if you're a part of the family, Berzerk if you're Xantiax-infected or a Vortex if you're stuck 10'000 years in the past fighting a pointless battle.