Your message should be:

Usually, it's a good idea to briefly explain why you decided to decline the job offer, especially if you spent a lot of time in the hiring process. There's no need to give a detailed explanation of why you made that decision. The best thing to do is to keep it short but genuine. Here is an example from The Muse: "While this position seems like a great opportunity, I have decided to pursue another role that will offer more opportunities to explore my interests.” Source: "How to Gracefully Turn Down a Job Offer" by Adrian Granzella Larssen on themuse.com Photo by Olya Kobruseva on Pexels