。
Because they really only come once in a blue moon.
Literal
Truly occasionally-[only-しか] come-not [reason-から] [agreement-ね].
The しか~ない construction ('only,' literally 'nothing but X not do') is Japanese's 'only'-with-negative pattern — where English says 'only come rarely,' Japanese says 'don't come except rarely.' たまに ('occasionally') + しか + 来ない = 'only come occasionally.' から marks the causal nuance ('because...'), and the final ね softens the statement and invites acknowledgment.