If he emailed a classified document, there is absolutely no way in hell that was an accident. You are never, ever, ever supposed to put classified information onto an unapproved system. This is like saying, "Oh, I forgot which pedal does which, so I slammed the gas instead of the brakes and killed someone, but it was an accident so I shouldn't go to jail." No, you're definitely going to jail, you 'forgetting' how to drive doesn't mean you didn't ruin someone's life.
(There's a small chance that he could have wrote something classified from his brain into the body of an email and sent it. People don't get sent to jail for this, provided it's a one-time incident. If he's getting charges pressed it was pretty egregious. The term "gross negligence" comes to mind.)
And it really could be that critical. Classified information is classified because it can be damaging. If you leak the names of people spying for you in foreign countries, they might be captured or even killed. How do you "design the system" to allow for that "mistake"?
Finally, the prison sentence lets people know that security is serious. There are many rules and it takes effort to follow them. People get lazy and cut corners when there aren't consequences.