Dortmund is not a weak team at all and even they got much more fans than you can even imagine in Germany. Compared to Bayern Munich and other teams where they spend lots of money for expensive players, Dortmund is a team that usually tries to rise and grow the players they need for the games and Dortmund is even a palyer supplier for many teams like Bayern Munich.
Quality players is everything a club needs, that's specifically what Borrusia Dortmund lacks at the moment, they keep selling star players that would have help them accomplish their goal. Bundesliga is competitive enough compared to other elite league in Europe. Bayern Munich and Borrusia Dortmund happen to be the leading competitive clubs that are able of reaching significant positions in UCL football. Borrusia Dortmund is measure on the same level with Bayern Munich when it comes to players, they have one of the best players in the team, the only problem they face is triggering the selling options for these players to elite teams and having no significant backup players.