That really depends on how the contract was drafted. Sometimes, it contains confidentiality clauses. It is really hard to name the reasons off the top of my head, because each case is different.
At the Office of the Commissioner, we encourage people who prepare contracts to be as transparent as possible and let the contractor know that the information is going to be accessible to Canadians, who want to know where their money is going and how decisions are made.
If a contract does raise national security issues, it is processed differently. However, the aim must really be to avoid a danger or a breach of trust.