When I had my exit interview, I genuinely came up with like, two or three things that I really did like about my particular firm and that we were particularly good at. I talked about how proud I was that we were so well regarded insofar as A, B, and C. “However, given that I want my next step to be more of 1, 2, and 3 (qualities that your new employer is known for, maybe), I think that this is just the right move to make.”
If you really want to put it on thick, you can say something like, “Believe me, if I wanted to stay in [things your current employer is really good at/known for], I don’t think there’s a better employer to do it for.”