The primary reason is to help clarify the saber in the third act when two blue sabers are exchanged and tossed all around against a heavy blue backdrop.

Especially with the inclusion of Leia capping off the sequence of ghostly voices, the purple color makes it clear whose saber Rey wields.

Carrie had indeed expressed a preference for that color when asked, when Leia’s lightsaber was merely a hypothetical.

Also, from a saga-only perspective at least, this saber was introduced and dispatched with in this film. Given the history as shown, it would seem there’d be minimal squinting required to have this change cohere well enough with future media.