The answer is simple: programming. If you make a bigger party you'd have to give more characters crowd-control (CC) abilities. Otherwise, you'd have to overpower the healer and tank so they can heal/tank through a 4-5 man pull instead of 2-3. Considering how impatienet the typical MMO player is today even the current dungeons are too long. Why would you program something that would take them longer to complete? So they chose a 4-man party.
