Why aren’t you earning more money at this stage of your career?

You like to make money, but other factors are even more important.

Example: “Making money is very important to me, and one reason I’m here is because I’m looking to make more. Throughout my career, what’s been even more important to me is doing work I really like to do at the kind of company I like and respect.

(Then be prepared to be specific about what your ideal position and company would be like, matching them as closely as possible to the opportunity at hand.