I know for me, where I get upwards of 30 emails a day, it's really easy for me to miss/overlook emails, or think I've replied when I haven't, or just plain take forever to get to them.
For me, personally at least, email can be a really, really bad way to get a hold of me. I know that there have been a few cases where people have been waiting on responses for me for about a week or so. :/ It's much easier for me to get things done in real time.
Only speaking for myself here, of course, but e-mail is definitely on the problematic side for me.