The Undisputed series broke with its biggest tradition in its 4th film, Boyka: Undisputed, but it was the right decision for the franchise. First getting rolling with the prison boxing movie Undisputed, the franchise didn't really take off in earnest until Undisputed 2: Last Man Standing, which moved the series into MMA. Undisputed 2 would also establish a novelty when it came to the protagonist of each film.
The first Undisputed featured George "Iceman" Chambers, played by Ving Rhames, as its main villain, with Undisputed 2 transitioning him into an anti-hero, along with Michael Jai White stepping into the role. Undisputed 2 also brought in Scott Adkins as the antagonist of the film, Yuri Boyka, before he was similarly moved into the protagonist's chair in Undisputed 3: Redemption. As one can infer from the title of the next movie, Boyka: Undisputed broke rank with the rest of the series by continuing Boyka's story. While this was a divergence from the tradition the Undisputed series had operated by, there are a couple of clear reasons for why.
The first was that the villain of Undisputed 3, Marko Zaror's Raul Quinones aka Dolor didn't give the series a great setup for its next anti-hero, nor was he ever meant to. Both Chambers and Boyka were prideful fighters who each needed their own lesson in humility. Both of them were misguided in their own ways and went through bitter defeats while battling corrupt prison systems, both coming out of it better men and better fighters. Dolor, on the other hand, offered no similar story arc.
As the fighter favored by the gambling crime bosses and prison wardens to win the tournament in Undisputed 3, Dolor had no reluctance to take performance-enhancing drugs in the scheme to rig the tournament in his favor. That's a huge contrast against how livid Boyka was in Undisputed 2 upon discovering that Chambers had been drugged in their initial fight without his knowledge, Boyka refusing to accept a win that was not on a level playing field and gladly entering a rematch with Chambers. Dolor's narcissistic and one-dimensional portrayal was also clearly deliberate, so apart from the real challenge he gave Boyka in the ring, he didn't offer anything like Boyka's inviolable coder of honor that could carry over into the next movie.
The other big reason why the series kept the Russian fighter as its lead is the simple fact that Adkins' Undisputed character Boyka was way too great of a character to not continue with. Far from merely being an excellent fighter, Boyka believes he's been chosen by God to become the greatest fighter to walk the Earth. Boyka summed it up with his immortal quote in Undisputed 3, "God has given me a gift, only one. I am The Most Complete Fighter in the World. My whole life I've trained, for what? I must prove I am worthy of something." With a mindset like Boyka's, the Undisputed series continuing without him was an obvious non-starter.
The Undisputed franchise set up a great gimmick of turning former villains into anti-heroes, but once Boyka entered the picture, the series had found the best hero it could ever have asked for. Boyka is not only an astonishing and truly formidable MMA fighter, but he's also frankly one of the best characters ever created in martial arts films. At this point, no chapter in the Undisputed series would be complete without The Most Complete Fighter in the World (or, for that matter, Scott Adkins).