The reason why is that the stage design rules are different for rifle, pistol and shotgun. Number of targets, type of targets, position etc.
It's difficult to harmonise them because the reality is that it's far more common to have a match that requires one gun, and it's also hard to harmonise them because in some regions certain types of gun are banned, so they aren't keen on restricting say, the shotgun rules to take account of the fact someone might use a pistol at the same time. Because pistols are banned.
The problem we had in the UK is that shotguns and rifles often cannot legally be even used on the same range because of the range approval criteria.
You would have a similar situation in Canada, because ranges are approved for restricted firearms and some aren't, but it's not quite as tough as in some other countries.
The legalities and the stage design rules make it very complicated.
If you ask anyone at IPSC I'm sure they'll tell you that it's for "safety" reasons, but it goes deeper than that, have a chat to Neil Beverley at UKPSA or Tim Andersen at IPSC Denmark if you really want it chapter and verse.