A capital is a rather artificial concept. Just because the lazy politicians decided to have their camp in a certain city that doesn't mean it actually has any economic power.
Agreed. And with the federal system in Germany being what it is, there are many government departments and offices which are not even in Berlin, which leads to even less demand there.
There problem is that there is almost nothing productive in Berlin, which means that there is relatively little demand for premium travel into or out of the city.
As for
totti's comment - there is one very important capital-to-capital service from Berlin: to Brussels.