Cristiano Ronaldo, as far as I can say and I think I know him pretty well over these years, does not make random decisions. And please note, that it is not something his Agent would suggest him to do. He makes such important decisions in life by himself still (also financial, withing family, etc). At first I was also suprised that he decided to move from RM, but when I noticed the club in decision was Juventus it started to make more sense.
1. Juventus is a big club with history- Cristiano likes to play for big clubs, so he can still see whether he can compete with top players in world
2. Juventus competes in Italy's Serie A - Cristiano likes to switch clubs in order to experience new leagues and top players in each league. His head is missioned always on victories and new challenges which he wants to overcome.
3. Juventus had great players play at their finest even if in older age - What comes to my mind is exactly this part that justifies this transaction. CR7 is 33 and in enxt 4 years he will be 37. This is considered quite 'old' in football careers for field players. However, I have seen Del Piero play by his 37 in Juve and he was great! There is also Buffon who just recently signed for PSG at his 40. Nedved was until 36 and perhaps if I'd dig deeper I'd find more world class players who 'aged well' in Juve. This is (in my opinion) one of the top reasons why he decided for Juventus.
4. Last but not least, Juve Fans - Maybe not many players look at fans while deciding to do a move. I also belive that in CR's case this was not the top priority, but I do know he appreciates great fans. By great fans I mean fans who are proud of the Club that they support. Maybe he missed that feeling in RM, because RM is popular in global scale due advertisements, marketing etc.. so it was hard to find pure RM fans. I belive in Juve this is different, The Old Lady has great fans who support her in many many years, truly, with heart.
EDIT: I forgot to mention also after-football-career. Since Cristiano would love to delve into fashion career after his sports one, Italy is a great place to promote also his fashion designs and CR7 brand in general. This might also had impact on the decision to join Juve.
So these are my reasonings why Cristiano transfered exactly to Juventus. I have to say, i am very happy to be here. Juventus is truly an amazing club. I will go continue to read about Juve's history and current status.