Is there any reason why Amala Akkineni did not play the wife of Nagarjuna in Manam (2014) instead of Sriya?

It always bothered me and I thought it made no sense to cast sriya ahead of Amala. I know Amala appears in the movie briefly. Although Manam is a full fledged Akkineni family movie, casting Amala instead of Sriya would have made the movie more 'whole' and complete. Did anyone else have the same question?

Samantha was casted because Chaitanya and her were already in a relationship by then and all of it made sense when they got married in 2017.