I can tell you why there may be some reluctance to do this. Basically it boils down to technology.
Why is it that Google+, Facebook, and Twitter are supported? It is because those three platforms (among others, I suppose), offer simple APIs that allows these kinds of things. When you click the share button through Google+, Facebook, or Twitter, it redirects you to a form supplied by one of those three services, which
- Checks if you are logged in: if you are, it proceeds, if not, it asks for a password.
- Now that you are in G+/FB/T's service, they take the information passed to them by StackExchange and form it into the rudimentary form of a post, waiting for you to fill in more details.
- You click send, or post, or share, or whatever, and it posts.
Note: nowhere in this process is StackExchange doing anything on your behalf. It tells G+/FB/T: some dude wants to share this link using your platform, you deal with it. And G+/FB/T says: cool, let me talk to him.
This is exactly not how e-mails work.
Remember there is no universal e-mail service provider. And neither is there a universal mail client. There just simply isn't a way of reliably using "some API" in the same way that G+/FB/T supports. Sure, on certain tightly controlled platforms you maybe able to do so (say Android or iOS), but this requires going through an operating-system interface to call up the default mailer etc., which on general purpose computers just don't exist.
So you are reduced to the same way News sites such as CNN.com lets you e-mail articles: you have to implement a service which asks the user to enter recipient address and sender address, and deliver the mail for the user.
Couple this to a near-anonymous discussion forum (this part is where it differs somewhat from the news sites, where they control the content of what you are trying to e-mail!) where users are allowed to enter their own content to be posted, I can just imagine the alarm bells about potential user/spammer abuse going off.