Having someone in heavy armor during a dungeon crawl doesn't make much sense either. The beasties would hear the knight ringing & clanging from 5 miles away... not to mention the knight would never be able to keep up with the light-armored companions. Heavily armored warriors dashing across the screen in games is always hilarious to watch.
In the quintessential dungeon crawl, the Mines of Moria in LOTR, the heaviest armor worn was ringmail by the dwarf.