Too much email spam from the company, including fake phishing attempts, to take my inbox seriously. It's a noisy channel.
Email is also slow. Instant messaging is where it's at, but most people's reading comprehension and writing are not at the level where that's the only viable medium.
Depends entirely on the context. There are plenty of cases where email, _especially_ when it comes to taking the time to sort out your thoughts and then format them nicely, is better than instance messaging.
