Because it also makes more sense for
Nevaeh. ae is pronounced AY. Or otherwise it would be neh-VAH-eh or neh-VAH-ay but not neh-VAY-ah. That would only make sense as Neveah, maybe that's why this alternate spelling is popular.
Heaven backwards makes an AY sound. Well, more like NEV-eh or so but with a longer eh so it sounds close to ay.