The main reason for this is the introduction and advancement of heavy armour, mainly plate armour and chainmail, from the 11th century until the promimence of firearms.

This kind of armor provided enough protection from slashing and stabbing to make shields obsolete. In comparison, gambesons and other types of padded armor provided good protection against slashing, but practically no protection from stabbing and projectiles (arrows and bolts). So, since soldiers could now be protected by their armor almost as well as with a shield, they no longer used shields as much. This allowed them to wield longer, heavier weapons like longswords, lances, poleaxes, warhammers and maces (the latter two were developed specifically to be used against heavy armor).

Plate armour was not used solely by kinghts, as depicted in pop culture. It became very widespread during the hundred years war in Europe. It's estimated that around 30-50% of participating soldiers wore this type of armor. The effectiveness of plate, combined by it's relative popularity in the late 13th and 14th centuries simply made shields unnecessary, so they were slowly phased out. Later, the promimence of firearms made all sorts of armour obsolete, including shields, and they were much less common begginning from the 16th century.