If they are waiting, we have to wait several additional months.
From a marketing perspective, I get it why they would want to have the biggest impact. More banks is better in the press. Also puts pressure on the holdouts (Sparkasse, etc.)
However, given the fact that Tim Cook said Apple Pay would come this year—which also has been reported by the press—I would guess that they would have to weigh the pros and cons here. Not being able to launch in 2018, in my eyes, is more damaging than saying a few additional banks are coming soon.
It's not like Apple is not known to just launch with a few and add other banks shortly after.