Security is the primary reason why. Thank the creators of viruses for screwing everything up for the rest of us. If you sent out any sort of big email attachment, nearly any corporate anti-virus/anti-spam filter is going to reject it, or remove the attachment. That's because there are so many different kinds of viruses and worms out there that try to propagate themselves via email. Those emails started out with simple attachments, trying to entice the gullible to click-to-open an attached application. As filtering evolved and that became harder, they eventually evolved to locking the virus payload up in a password-protected ZIP file, with instructions and password email written out in the body of the email message. You might think that people wouldn't be dumb enough to open something like that, but you'd be wrong. Actually, some of the deceptions are quite complicated. They look like emails from somebody you know, and you might even think that your friend is sending you something you want.
Even if the attachment doesn't get blocked, smarter recipients are wary of big attachments. Some will suspect the email is a virus delivery attempt. They'll question the email, ignore it, or report it as spam.
Also, email size is a concern. Big email messages can choke the recipient. That big attachment would take up a lot more space on the email servers between the sender and recipient. If the recipient was on a slow connection, it will take a lot longer for them to download the email message. They could be stuck downloading the attachment, even if they didn't want to. Not everybody has broadband yet, so you would not assume that your recipients can handle a giant-sized email.
Think of how HTML email messages work: When a legitimate email service provider serves a rich graphical email with HTML and images, they're not really sending all those images directly inside of the email. None of the images are actually embedded in the email. The HTML source code just links to them, usually back to the email service provider's website. If all the images were embedded, the email would be pretty large - but this way, the email itself is only a few kilobytes in size.
Do the same thing when you need to share a file with recipients on your list. Instead of attaching the file, link to it in your email. You do that by hosting it on a website, then pasting the link into your email message.
Don't have a website to host it on? Try Google Pages. This probably isn't the best way to share a file with hundreds of recipients, but if you're sharing it with just a few folks, it's very easy to set up. Creating a Google Pages account gives you a website at (your handle)