There is a software company and they want me to interview with their manager and they would let me know if i can go to the next stage?

I have worked in a large company with 3000 employees and they had the same approach and I felt it was not a very good fit for me and I left them after 1 year and I was sure that I did not make any mistakes. The interview went well with the manager but I had to leave them as I was not feeling good and I was only making 15% more. The interview went well again with this company and they were ready to offer me 30% more than my current role and I don't want to go to the next stage. They are asking me why I don't want to go for the next stage and I was wondering what should I tell them?