Because it has been defined this way…
A logical explanation : it is because the UL is the limiting path in GSM, so by using lower frequencies, it is actually possible to increase the UL level a little bit more.
High frequency takes more power to transmit the signal low frequency comes under the level of down link mobile having a small power back up source but our base station haveing enough power source like d.g , Battry , A.C Power supply.
so we are choose up link is low frequencies, Eg in satelite up linke used high frequencies.