No direct reasons were given, though it thankfully seems like this decision was made in an amicable mindset, and there won’t be any bad blood when she leaves.
For the most part, Arianne Zucker has been a part of Days of Our Lives ever since 1998, though she did leave the show in 2006 before returning in 2008.
(Which, of course, could lead to her reprising the role of Nicole Walker in the future, assuming the show doesn’t kill her off.)
She has been as much a part of the series as everyone around her, even the longer-lasting vets, but it appears she won’t be seeking to renew her contract whenever it expires in April.