Because it was a negotiated peace at the end of a limited war, not a victor's diktat as was common in the twentieth century. The British had failed to achieve their objective of restoring control of the Thirteen Colonies; but they still controlled armies, navies, industries, and the ability to inflict a lot of pain on the French. A peace negotiation seeks a treaty that the defeated party prefers to fighting on; demanding Canada would have caused the British to continue the war - and who knows how that would have gone? The Americans did try to invade Canada at one point, and failed miserably. Austria or Prussia might have come in, and turned the conflict into a general European war, and those are very risky for Continental powers. French finances, as always, were in disarray, in fact they were about to cause the French Revolution. In short there was every reason for the French to quit while they were ahead, and not double down.

There's also the question of whether they even wanted Canada. The fur trade was pretty much gone, and the Seven Years' War had demonstrated that Quebec could not be defended. No economic advantages and a military liability; why would they take it back? The sugar islands were clearly a much more important prize.