The reason for this is simple, the laws of guns in Canada is EXTREMELY strict, nothing close to the gun laws of the states. Too many kids are irresponsible with airsoft guns in Canada resulting in serious injuries causing the law to become more strict over time.
The current law is unless you have a gun licence, Airsoft guns must have a clear or tinted plastic upper and/or lower receiver. The receiver is basically the handle part of the gun up. This is for rifles, and basically the same for pistols. For sniper rifles the law is they must be plastic, but don't have to be clear.
An example of a Canadian Legal Airsoft Gun:
This is an pretty extreme example, here is an example of a BARLEY Canadian legal airsoft gun:
All image credit to buyairsoft.ca