The long A sound is pretty popular right now. Names like
Katelyn,
Aiden (and all its friends, like
Jayden,
Brayden,
Caden, Zayden etc.), Kaylee/Hailey/Braylee etc.,
Grace, Kayla/Mikayla/Layla have been popular for the past couple of decades.
Ava fits into that category well. Also A names are popular (
Abigail,
Ariana,
Adelyn,
Addison,
Aliyah,
Amelia etc.). Names that are popular that start with E have the short E sound (
Emma,
Ella,
Emily,
Evelyn,
Ellie,
Eleanor). The most popular name to start with long E in 2016 was
Eden, clocking in at #186! So, i think the short answer is that
Ava follows current sound trends, whereas
Eva doesn't as much.