Not sure of the reason, but it has been confirmed by Universal that Diagonal Alley will not open during their Halloween event Halloween Horror Nights, which is from Septermber 19th to November 1.
The reason is unknown, but I guess because the event already gets the park busy, they don't want guests that are just coming to visit Diagon Alley.
Perhaps its because its such a confined space compared to the rest of the park? Halloween horror nights could cause serious over crowding in that area... especially Knockturn alley, which is the "spooky" section.
So they'll have to put attendants at the entrance to Diagon alley, which cost money and will take attendance away from the haunted houses.