Maybe because he knows that in Porto the moment he creates a good team, half of those players will be sold in the summer and he'll have to start all over again, while the richer Juve can allow him to create a team and use it for much longer, adding new pieces to the team from time to time. Hypothetically speaking, of course.
In reality he can do that in many other big teams, where he'll have both bigger budget and more chances to succeed.