One thing that I have to mention is that both Inter and Milan team are the strongest in recent years so let's not give credit only lack of performance of Juve.
Again, I feel the main reason is, Ronaldo COSTS A LOT, Juventus thought by just having him, they would win CL, did not realize while Ronaldo is an important piece of puzzle, he is never going to win anything on his own, he NEEDS good players around him, and unless you are a rich club, very hard to have Ronaldo surrounded by decent players.