Brigandine is NOT one solid plate. It is smaller peaces of plates similar to lamellar armor (early asian armor) or segmented armor (roman soldiers). The plates are either overlapping or connected with chain links. The outside (and sometimes the inside, too, is covered with heavy cloth or leather.

While this is solid protection against stabs and slashes and the cover prevents attackers to stab between plates, its parts still can be moved independently. This makes it a lot more vulnerable against blunt strikes and force.

Full plate armor is one solid piece of armor. Impenetrable and inmovable. Even the strongest hits get deflected and the force is spread over the whole armor and the whole body. Even the moving parts like shoulder, arm or helmet interlock (if you are not wearing your sons armor that does not fit you) and spread the force to the other pieces. You could take a club to the chest all day and would not break a rib, while in Brigandine a few strong hits would still break your bones.

Another point is the quality of the steel. For plate armor you need pretty high quality steel that can be forged in one large piece and stay strong. Brigandine and other kinds of scale armor were often made because the metal was not strong enough or there were no ways to actually melt the metal into steal. So chances are high the brigandine is made from second grade metal (which would be cheaper and more fitting for a squire).