Mainly because the 45 Colt case is shorter and if you shoot a lot of 45 colt and don't clean, it may make a constriction ring (carbon and lead buildup) where the mouth of the 45 Colt case sits when it's fired. If you shoot 454 afterwards, with the longer case, it may create excess pressure due to the constriction ring.
And FA 83's have tight chambers.
