If the Kansas City Chiefs needed a reminder of how difficult it is to repeat as Super Bowl champions, they needed only glance across the field at Tampa Bay Buccaneers quarterback Tom Brady.

Brady won a record seventh Super Bowl on Sunday night as the Buccaneers dominated Patrick Mahomes and the Chiefs on their way to a 31-9 victory in Tampa, Fla.

He has appeared in 10 of the last 20 Super Bowls. But he has won two in a row just once, in the 2003 and 2004 seasons. Like the New England Patriots in the 2017 season and the Seattle Seahawks in the 2014 season, the 2020 Chiefs failed in their bid to win a second straight title.

“I think what makes it such a challenge is it is hard to win one Super Bowl,” Brady told reporters last week. “You cannot go buy a football team. You have to develop players.”

The Buccaneers, coached by Bruce Arians, had a lot of young talent, but their roster was largely constructed in the off-season when they signed Brady, tight end Rob Gronkowski, running back Leonard Fournette and others.

The Chiefs, though, seemed destined to repeat. They finished the regular season with an N.F.L.-best 14-2 record and were favorites heading into the Super Bowl. Their offense looked unstoppable with quarterback Mahomes, last year’s Super Bowl most valuable player, playing well.

But before the game, Mahomes acknowledged that parity in the N.F.L. made it difficult for teams to repeat as champions.

“I mean, literally, you could be the worst team in the league one year and work all the way up to the Super Bowl the next,” he said.

The salary cap, which limits how much money teams can spend on player contracts, is a big reason for that parity. That wasn’t the case decades ago. The Green Bay Packers won the first two Super Bowls in the 1960s, and the Miami Dolphins and the Pittsburgh Steelers pulled repeat wins in the 1970s. (The Steelers did it twice.)

Quarterback Joe Montana led the San Francisco 49ers to consecutive titles in the 1980s, and the Dallas Cowboys were repeat champions in the early 1990s. But since the N.F.L. introduced a salary cap in 1994, only the Patriots and John Elway’s Denver Broncos have repeated.

As is often the case in the N.F.L., injuries can derail teams in an instant. The Chiefs struggled on Sunday, in part, because they were missing their two starting offensive tackles, including Eric Fisher, one of the best tackles in the game, who missed the Super Bowl after tearing an Achilles’ tendon two weeks ago.

Laurent Duvernay-Tardif, an offensive guard who has a medical degree, opted out of the 2020 season to work in his native Canada to help combat the coronavirus. Tackle Mitchell Schwartz played only the first six weeks of the season before injuring his back.

The Chiefs also faced a last-minute coaching change. Their outside linebackers coach, Britt Reid — the son of the head coach, Andy Reid — missed Sunday’s game after being involved in a car crash in Kansas City, Mo., on Thursday night.

The Buccaneers took advantage. They sacked Mahomes three times and pressured him on 29 of his 56 drop backs, according to ESPN Stats & Info, the most in Super Bowl history. Mahomes spent most of the game scrambling from defenders behind the line of scrimmage. He threw two interceptions, no touchdowns, and needed 49 passes to accumulate just 270 yards, most of them late in the game.

Retaining a roster that has made it to three consecutive A.F.C. championship games will be difficult. The Chiefs are almost $18 million over next year’s salary cap, according to Over the Cap, an independent site that tracks N.F.L. contracts and salaries. A number of key players are free agents, among them receivers Sammy Watkins and Demarcus Robinson, center Austin Reiter and defensive backs Daniel Sorensen and Bashaud Breeland.

The Chiefs will face another hurdle: The salary cap, which is based on the league’s total revenue, was about $198 million this season. It could fall to as low as $175 million next season because the league lost billions of dollars in ticket sales during the pandemic.

For their part, the Buccaneers will have an estimated $28.9 million in cap space, which will give them room to re-sign players and attract free agents.

The Chiefs will, however, retain Mahomes, a transcendent quarterback who signed a 10-year contract last summer worth up to $500 million. At only 25, he has many years ahead — barring injury — to match Brady, Elway, Montana, Troy Aikman, Terry Bradshaw, Bob Griese and Bart Starr as quarterbacks who have won back-to-back Super Bowls.

After the game, a downtrodden Mahomes acknowledged the difficulty of winning in the N.F.L. “When we joined together we knew it wasn’t going to always be successful and we weren’t going to be able to win a thousand championships in a row,” he said. “We knew we would go through times like this, through adversity.”