it was simply chosen by the creators. It don't matter who it is and why, is just the circumstances.There are different champions in every Pokemon version .
Sadly, alder.
When you first beat the Elite Four, you will find that N has beaten the champion Alder, but after beating N in his castle, he will fly away on Reshiram and Alder will return to being the champion. But I personally (others agree) think that N is the champion. After all, he did beat Alder. But if you want fact and not opinion, Alder is the champion.
Alder
Alder is only in pokemon black or white.
Most of his Pokemon are bug type.
The champion which is alder has accelgor, escavlier, druddigon, vanilluxe, boufallant and volcarana
you dont battle alder n becomes the champion so you battle him then (if you beat him) you battle ghetsis.
Alder the champion will give it to you on Twist Mountain.
Surf is given to you by Champion Alder at Twist Mountain
The League champion in the games are N, the Team Plasma king. Also there is Alder the Pokemon black and white champion
Alder, a man with fire red hair. You will see him in your game a few times.
The Champion in Pokemon Black and White is only after you have defeated N and Ghetsis For the final time. Then the second time you battle the Elite Four/Pokemon League, You will battle Champion Alder. All his Pokemon are Lv 75.