Smart decision by the authorities.
The Inter fans will complain that FIGC is helping Juve, while it is obviously helping Serie A and keeping things fair. The Derby, which in this particular season should be the main sporting event of the day in the entire world, was facing strong competition from Real-Barca, which was scheduled at the same time. How many neutrals would go for a Juve-Inter played in an empty stadium when Real-Barca is on?
The other reason, or the most important one, is of course the fact that in a close title race you can't give such an advantage to one of the teams, by forcing the other one to play its home game behind close doors and lose money.
The same should (and would) have been done if the match was to be played in Milano.