Except Logic may not be telling them that they need to join rather than join. They may see no pressing need
Remember, the whole reason Ni'Var seceded in the first place was because of the Burn.
Since it's been proven that 1) the specific cause of the Burn has been found; 2) the Federation was not responsible; and 3) the Burn cannot possibly repeat itself ... there's no logical reason for Ni'Var NOT to rejoin the Federation.
