Why are you leaving?

I have a job interview tomorrow morning for a position I'm very excited about that would be a big step forward for my career. I've been thinking about my answers to potential questions and I'm anticipating being asked why I want to move on from my current position. The truth is, I'm looking for a new challenge and enjoy building things, the position is for a new project with an existing organization. Does it sound flaky just to state that I'm ready for a new challenge and the next step in my career? I anticipate being asked why I think I can't move on in the same way at my current position and honestly there are both personal and professional reasons for why I feel like it's time to move on. Would it be enough to be honest and say that I don't think my current employer is financially stable right now given layoffs of high-ranking staffers? Our Chief Development Officer was let go a few weeks ago and it doesn't appear that the remaining development staff is having any better luck without them. I don't want to get into the muck of the personal reasons of why I'm moving on because I know that's unprofessional and don't want to trash talk my current employer or manager even though I'm looking to move on.