They lost Super Bowl IV because the NFL needs its underdog/feel good stories that they can spam for decades to come. Not to mention it helped launch NFL Films into stardom with their audio of head coach Hank Stram.
The others were indeed lost because the AFC had the hotter teams at the time.
Of course, little known tidbit, but many would argue that the 1975 Vikings were actually the best roster/team of their run in the late 60s and 70s, and just got upset in the divisional round thanks to a no-call offensive pass interference, which would conveniently go unknown for years until about the mid 2000s when the media began bringing up the history of the Hail Mary in the NFL, and they were forced to acknowledge that the first-ever "Hail Mary" was actually a penalty.
Had the Vikings won that game as the 1 seed in the NFC, it probably would've them, and not Dallas, facing Pittsburgh in the Super Bowl, which would've given them a 4-peat (even if it meant them doing what Buffalo would eventually do a few decades later).
