Because its rumored that (and this is a rumor forum) email was/is the answer to send pictures to another recipient which is very easy to do on the iPhone. The other side was taken over with SMS and that is why. Currently no one knows what SJ will do on iPhone 2 in regards to your question, so therfore can't answer your question with any other promises.
It clearly will be possible once apps are released in June..all it needs is access to your pictures folder and know the apn of your mms service and hey presto you can send and recieve mms.
You already can if your iPhone is jailbroken, just install the MMS app.
It will NOT be possible through the official SDK as App Store apps cant use the connection to the carrier which is what you need to do to send MMS messages.
Actually, almost any phone you can buy today has e-mail funtuality, but the mobile internet plans and the e-mail apps on these phones are so crap almost no one uses them.