I think your question is a good one. From what I know, JetBird has strong backing from an Irish investment group and a group of Saudi investors (see here:
Fly JetBird - Media), but, like most companies these days, going slower than faster is preferred. Perhaps this operation will have to prove itself before it gets further funding for expansion - basically investment "stage gates." It will need to prove itself. I don't know for certain what is happening but I understand their prudence in this economy.
Personally, I think Cologne is a nice city but I would rather live elsewhere - AMS, London area and others. I hope JetBird does well enough to expand pilot domiciles. Again, it looks like an interesting concept and a very nice aeroplane but I wish the economy were stronger to facilitate growth. The down economy does concern me and it puts this operation and other start-ups (i.e., Jet Republic) at some risk. Let's hope that the Irish investment firm maintains its commitment and does not cut the cord too quickly... It could be a good long-term play.