It's a good question.
You can google the Treasury Board Secretariat rules to explain the differences between enhanced reliability, confidential, secret and top secret, but in a nutshell, here's the easiest way to break it down.
In my estimation, 95% of the intelligence that needs to be shared with any government department or parliamentarian is never above the secret level.
The difference between secret and top secret.... Top secret is how we got that information and the need to protect the sources, whether that's people, techniques or whatever. Who cares? We need to know what the crux of the information is . That's why secret is the appropriate level.
Again, the resources required to communicate at a top secret level are very costly. I can go on at length about where we lack on classified communication systems in this country if it comes up in another question.