That's a good question. People might say you have it in the Constitution, but the courts might still not be able to enforce it. Well, I would take the view that there are some provisions in the Constitution--I know there are--that are there very clearly and arguably aren't enforceable either.
Picture it. You have the Constitution very clearly in an article setting out the circumstances under which prorogation can be sought or prorogation can be given. I find it very hard to believe that the Prime Minister of the day is going to disregard something that is as clearly written in the Constitution and seek a prorogation. Not only that, a fortiori, it's not likely that the Governor General of the day is going to grant a prorogation that is recognizably contrary to what's in the Constitution.
There comes a point at which the Constitution sets the rules and the major players acknowledge that: you don't need to worry about court enforcement; it's simply not going to happen. Were it to happen, contrary to the Constitution, then you have a real constitutional crisis, and any court order might be marginal at best in those very extreme circumstances.