No team has won back-to-back Super Bowls since the Patriots in 2005. Why has it been so difficult?

Super Bowl MVP Deion Branch hoists the Lombardi Trophy, flanked by Bill Belichick and Robert Kraft, after the Patriots beat the Eagles in Super Bowl XXXIX in 2005.Chin, Barry Globe Staff

This story originally appeared in Point After, the Globe’s Patriots/NFL newsletter. Sign up to receive it in your inbox.

Back-to-back Super Bowl winners used to be a fairly regular occurrence in the NFL. These days? You’d have better luck finding video of Bigfoot riding a unicorn.

In the first 35 years of the Super Bowl era, there were seven back-to-back champions: The Packers (Super Bowls I and II), Dolphins (VII and VIII), Steelers (IX and X, then XIII and XIV), 49ers (XXIII and XXIV), Cowboys (XXVII and XXVIII), and Broncos (XXXII and XXXIII).

But since the Patriots beat the Panthers in Super Bowl XXXVIII and the Eagles in Super Bowl XXXIX, no one has managed to win consecutive Super Bowls. It’s the longest stretch of the Super Bowl era. (By way of comparison, the NBA and NHL have both had at least one set of back-to-back champions since 2003-04. MLB has not.)

Now the Chiefs have a chance to make a little history; If they beat the Buccaneers Sunday, they’d be the first team since those New England teams to win back-to-back Super Bowl championships.

So why has it gotten more difficult to repeat? A few reasons:

While every team and every year is different, it’s hard not to notice that only two franchises — the Patriots and Broncos — have gone back-to-back since the inception of the salary cap. (Even though Denver was later penalized for cap violations.) In an era of planned parity, cap limitations, and balanced scheduling that’s designed to bring everyone to the middle, it’s now harder than ever to replicate a championship.

Can Patrick Mahomes and Andy Reid lead the Chiefs to back-to-back titles?Gail Burton/Associated Press

You also need good timing, at least from a team-building perspective: With New England and Denver, offensive stars like Tom Brady and Terrell Davis were in the early stages of their careers, which meant they were relatively cost-effective at the time, which allowed the teams to blend them with established (and more expensive) veterans.

Consistency and continuity also play a role. In a copycat league, success means your team, coaching staff, and front office will be poached at every opportunity. The Patriots ended up losing coordinators Charlie Weis and Romeo Crennel after the 2004 season, but both New England and Denver kept the fundamental elements of their coaching staffs together (head coach and both coordinators) in the runup to their championships, as well as both seasons of their back-to-back titles.

Finally, there are things that can’t be quantified that every great team needs — luck and good health. New England suffered some astounding personnel losses both years (Rosevelt Colvin and Ty Law both missed extensive time in 2003 and 2004, respectively), but still held its core together. Denver played four games without John Elway in 1998, but for the most part, the Broncos stayed as healthy as possible through that stretch.

Ultimately, the Chiefs have been able to follow the blueprint laid down by Denver and New England all the way to the cusp of a second straight title. Kansas City has maximized its financial flexibility, kept its core together, and thus far hit on the intangibles. So can the Chiefs finish the job? We’ll find out Sunday.