Largely due to logistics and location. They weren't so much chosen to liberate the Netherlands, as their position in the line, made them the ones to do it.

Basically since the capture of Caen the Canadians held the position of the left flank on the Allied front. Thus through the liberation of France the Canadians advanced along the coast. After the (primarily British) thrust toward Arnhem, the Canadians were tasked with clearing the Antwerp approaches, being the ones on that side of the line. When the time came to advance into the heart of the Netherlands, the Canadians were still there on the far left, where they had been since Normandy. So the Canadian First Army, being on the far left, was again given the left most target, the Netherlands.

This map shows the maneuvering of the Allied armies from the breakout of Normandy, until Operation Market Garden. You can see how the Canadians advance along the coast.

This map shows the Canadian advance into the Netherlands. You can see how they are given this because of their position in the line, which was assured due to the earlier line of advance through France. The British Second Army advanced toward Bremen. The US Ninth army advance to the right of them. Each army simply moved toward what was in front of them.