We started with the plan of making one app that does both, but as we dug in, it became apparent that doing so would lead to a lot of compromises that would make the app much harder to use. Some contacts have email addresses, but no phone numbers, and vice versa. If you added someone for messaging that doesn’t have an email address, we wouldn't be able to send an email to that person. When you’re creating a contact or group in the app we would have to have a complex process to select which email/number you want to use for messaging and which one you want to use for email. Some users have different email addresses for iMessage and for email, which we would have to account for.
Based on our experience, we think that purpose-built apps for messaging and for email make both much easier to use and our users much more productive as a result.