Aside from the fact that he probably didn't get the choose the armor when it was first put on him, there's several possible reasons.
1) <PERSON> most likely didn't think he needed any extra protection. One of the biggest faults of the Sith are that they are overconfident in their own abilities. With his newly found Dark Side powers, <PERSON> felt invincible, armor or not, which previously had led to him attempting to approach <PERSON> from the lower ground and getting the last of his limbs removed. Despite this defeat, <PERSON> was so confident that he would not lose another fight, he simply chose not to take the extra precaution.
2) It's possible that wearing a full set of Beskar would simply be too bulky. Even Mandalorians only wear helmets, chest pieces, and shoulder coverings. 865
From what we see of <PERSON>'s fighting style before dawning the suit, he much prefers agility and dexterity over being able to resist a few lightsaber strikes. Remember, the only thing preferable to armor protecting you from a lightsaber is not getting hit by a lightsaber in the first place. More likely than not, <PERSON> preferred to rely on his own fighting skills as a way of defense, rather than go the more "tanky" route.
3) Additionally, it's quite possible that <PERSON>'s armor is already efficient enough. On Cloud City, we not only see <PERSON> block <PERSON>'s laser blasts with his gloves (which some sources say are actually made of Beskar), but we also see that his armor prevents <PERSON> from further damaging his arm when <PERSON> fails to deflect a lightsaber strike. <PERSON> may not be <PERSON>, but it does seem to have some effect on lightsaber blades and blasters.
Lastly, let me pose a counter-question. If <PERSON> is so great, why would every Jedi not be dying to get their hands on it? Wouldn't it provide a distinct advantage to the wearer in any lightsaber fight? Whether it be for one of the above three reasons or a different one, there definitely seems to be some logical reason that Jedi and Sith alike prefer their cloth robes to battle armor of any sort. 503