Well, they are easy to eavesdrop with, especially if you have some technological know-how! That I am lacking in, but I have heard others speaking about "hacking" conversations, to listen in. Plus, they require the communicator to be with you at all times, and I certainly don't bathe wearing mine, or sleep with it on. They can thus be lost, or broken, and important messages might go unheard.
So you see, for all their convenience, sometimes mail is absolutely critical to ensure, without a shadow of a doubt, that the message has gone through!