The easiest way to look at it would be even if Dortmund sold no-one this summer, would they be in the top six favourites to win the Champions League next season? Probably not. Whereas Bayern will always be in the top three contenders at the moment, and there's no way anyone would back against them to retain the Bundesliga next season.
It's hard to even come up with a comparison point against another country, there isn't anyone near Bayern's level as far as club stature and finance goes in Germany, not even close. It's a cross between say, Basel's current on field superiority in Switzerland (won all the title in all but four seasons since 2001/2) and how much bigger in stature Celtic/Rangers are than everyone else in Scotland.
Dortmund had their best ever team and best ever manager 3-4 years ago. No chance of making it stick and even challenging to become the dominant force, they got picked off as Bayern are just miles bigger in every way.