A lot of the fancier designed helmets in the medieval period were more for tournaments, jousting and show rather than actual combat. On top of this Knights generally fought on horseback this negates two of the main disadvantages of a horned helmet, namely susceptibility to grappling and preventing the deflection of blows from above. On horse back a knight is unlikely to be grappled on the helmet and even less likely to receive a blow from above, therefore a mounted soldier would have less of a disadvantage compared to a Viking who fought mostly on foot.

My favorite example of a horned tournament helmet is one owned by Henry VIII of England the design was impractical for combat so was most likely for show in parades or pageants.

Many great helms were designed to cover a smaller helmet underneath, either a cervelliere or a visorless basinet, so It was entirely possible for a knight to wear an impractical Helmet for show but remove it if he had to resort to close combat.