Because Dortmund is honestly only one of a select few in Bundesliga that can beat Bayern. Bayern is going to be 1st in that league for years to come and that is the only league where you know who will finish 1st every year.
Liga BBVA: Real Madrid, Barcelona, Athletico Madrid
Ligue 1: PSG, Lyon, AS Monaco
Serie A: Has so many teams
England:Chelsea, Man City, Man Utd, Arsenal can all challenge for the title, but then there are teams like Tottenham, Liverpool, Southampton, Everton who can challenge for top 4.