the reason of this is very simple
admin who banned you got 1 demo or ss or proof about ur ban, after some time (7 weeks according to rules) admins can delete all that proofs from his pc
so, if 1 cheater is banned today, but he post the unban request in 3 months, OFCOURSE the admin who banned him will not keep the demo 3 months on his pc..... for this reason you should post unban request in the same day u are banned