1) He doesn't need the money
Certainly Newey has a pretty good contract at Red Bull. The team are also keen to keep him and can afford to counter any offer Ferrari make. Currently he is rumoured to earn $10m dollars a year so paying the mortgage is not keeping him awake at night!
2) He doesn't need the hassle
Ferrari is a religion in Italy. The pressure is monumental, the commitment needed to succeed there is far bigger than any other challenge Adrian will have encountered. He will also have to completely restructure the technical team to 'his way' and be allowed the freedoms currently offered by his Milton Keynes based employer. What if he does it and it doesn't work out? It will be the only blot on an incredibly impressive career. Its a huge gamble. A massive risk.
3) He doesn't want to move to Italy
A simple point. Adrian has said many times that he doesn't want to uproot his family and move to Italy. The only way he could stay in the UK is if Ferrari opened a UK design office like they did back in the day with John Barnard designing the cars in Britain. Whilst this might be easier these days with the power of technology and instant communication, whilst other teams are finding that doing everything in house is by far the best option (Mercedes), it seems like it might not be the route to success for Ferrari.
4) It's not his dream job
Adrian Newey has been quick to point out that his next dream project is to design an America's cup yacht. It has been widely reported that Sir Ben Ainslie wants to get Adrian involved in his attempt to win the America's Cup with a British team, of which his plans are being released very very soon!
5) Why leave the team you feel so comfortable in?
Adrian has it all at Red Bull, He has a team completely built around his way of working, and the freedom to pretty much do as he pleases. He clearly enjoys what he does and where he works so why would he want to change? The grass isn't always greener!