Structural strength ! You need heavier body to absorb the recoilling force of 125 mm gun and this tank is only 35 ton Plus the length of the barrel will hinder the operation of the small tank. This tank was meant for either jungle and swamp warfare in the south China or mountaineous area in Tibet highland having longer barrel will be a handicap because of narrow and inclined road , twist and turn of mountaneous road, etc
Hmmm... But the Italians are mounting 120mm gun on Centauro TD which have similar weight and the disadvantage of being wheeled... I guess the PLA just don't bother developing an all-new soft recoil 125mm gun. I can't really comment about the barrel length thing, but if the 125mm is longer than 105mm then I guess it does make sense.