Bayern Munich did not become German champion for many years because they are so good. They often became German champions because the other teams were worse and did not perform consistently well throughout the season. There are 34 matchdays and the players from Bayern Munich have the mentality to take their chances until the last second. Only when the whistle blows after the 34th match day the game is over for them.
I don't think it's mostly about performance, because at some point Dortmund had quality players and has produced top class players but they sold them off to other clubs for cash benefits, they would had been a very tough team and competed with Bayern to win the trophy, Bayern Munich is more successful than them, because they invest alot on quality players and use them to achieve trophies and sell the players when they're incompetent unlike Dortmund that sell players when they're at their peak.
Well so far Leverkusen is improving and with how they're going they may likely be the team that would stop Bayern's dominance in the Bundesliga over the years, though it's not going to be easy winning a team that's dominated for decades, Dortmund tried and almost got it but failed let's see if Leverkusen would.