It might be because he is already dead(which I believe is true).
Or, He has an access to a million bitcoins. Imagine a million bitcoins coming back to circulation. Demand will be the same and supply will increase. But Bitcoin runs on speculation, or so they say. People might not think Bitcoin is a safe investment and might dump all their coins to keep their investment "safe" which in turns drops the price significantly and makes the market unstable.
But satoshi refuse that he is the founder of Bitcoin in many interviews. I think founder might be from Australian region and as said he is hidden due to Bitcoin seems to be decentralized. Once it become centralized he will reveal that in public automatically.
I never heard about satoshi refusing to be the founder of Bitcoin.