Because Kishimoto feels the need to kill off all people who were above Naruto/Sasuke so that they would stop progressing and Naruto/Sasuke would just surpass them. It's just plot.
Examples:
Naruto
Nagato-committed suicide in order to revive everyone because Naruto told them they share the same sensei and showed him a book
Minato-committed suicide to seal away Kurama instead of resealing him back into Kushina
Sasuke
Itachi-got some random plot disease that resembles tuberculosis (not aids) and planned his own death.
Itachi's time in the story is over. Just let him R.I.P.