The problem is for a long flight with the fuel in the wings becoming very subzeo. You can then land in even tropical conditions and have ice on the top of the wings. And certain places have no deicing. So you vave to wait for the fuel to become positive or the sun to shine. I fly the T7 and landing with more than 20 tonnes in a tropical country would put you in the above case. (And imagine deicing in HAV or BZV...)