It's been longer than that. I am 37 and the 80s were even worse than the 90s for blowout Super Bowls. It's a combination of factors.
1) Better preparation. There's so much better information on players, schemes, and coaches nowadays that a team would be hard-pressed to come into a super bowl without being prepared.
2)Free agency. Back in the day, team gets good and keeps its players until it decides to get rid of them. Players had little power/control over their situations, so teams could stash good players on their bench/practice squad. Nowadays, a team gets good and if they aren't smart, they can't lock up their mediocre/role players typically. It forces them to constantly swap out parts. Less continuity + free agency = no true dynasties. Yes the Patriots and Ravens and Steelers keep winning, but they are also constantly swapping out parts around a few stalwarts.
3) Randomness. Since the gap between the most talented playoff teams and the next level has narrowed, the 1-and-done format of the playoffs leads to a greater chance of the 'morally' best teams getting upset. It's not a best-of-7.
There's probably more reasons. That's off the top of my head.