To answer this question we first have to adjust the opening assumption that the entire royal family was evacuated to Canada. While princess Juliana and her daughters Beatrix and Irene were indeed evacuated to Canada, queen Wilhelmina and prince Bernhard - Juliana's husband - remained in England. Apart from ensuring the safety of the succession to the throne, the princesses had no active part in the Dutch government (Beatrix and Irene in particular, they were still children) and as such it was deemed better to evacuate them from England as it was not at all a given that it would be a safe location; as soon as May 20, the president of the United States, Roosevelt, had telegraphed the queen that in the eventuality of German bombings of England, he was willing to send American ships to Ireland to facilitate evacuation of the royal family to the United States.^(1) Instead, the Dutch East Indies (Indonesia) and Canada were considered. The former was less than ideal: not only was it a long distance away, but even in 1940 the political situation in the far east where it concerned Japan was uncertain. Canada was considered the best option.
The reason was that Canada - while overseas and out of feasible reach of Germany - was still a part of the British Empire, which, just like the Netherlands, was in a state of war with Germany. This ensured that, as Wilhelmina would later put it:
"alleen dáár zeker was dat zij (Juliana) de koninklijke macht kon uitoefenen als in Nederland"^(2)
"only there it was sure that she (Juliana) could execute her royal powers as she would have in the Netherlands"
In other words, the state of war between the British Empire and Germany ensured that Juliana - if necessitated by her succession - could execute her powers as Queen of the Netherlands without interference from pro-German elements. While that answers why Canada was a better choice than the United States (which at this point had not yet declared war on Germany) it doesn't explain why the Dutch colonies in the west - the Netherlands Antilles (including Aruba) and Suriname - were not seen as feasible alternatives.
The political situation surrounding the Dutch West Indies was complex. It was a colonial vestige in a part of the world that, due to the anti-colonial attitude of the United States, was seen as an anomaly, especially as Roosevelt was eager to maintain good relations with the various Latin-American republics that had wrested themselves from colonial control in years past.^(3) Suriname in particular garnered attention from the United States, as it was their primary supplier of bauxite (used to create aluminium), yet was ill-defended by the Dutch (the United States would when war was declared on Germany garrison it with troops, primarily because the government in nearby French Guyana was loyal to Vichy France).^(4) Of the aforementioned Latin-American republics, Venezuela was eager to exert their influence over Curacao and Aruba (due to an incident in 1922 in which Venezuelan insurrectionists had captured a ship in Willemstad in Curacao that carried the governor, Venezuelan-Dutch relations had remained tense in the first place). Coupled with the anti-colonial sentiment of the United States, it was an unpredictable state of affairs and definitely not considered to be safe grounds for the princesses.^(5) Princess Juliana would on occasion visit these colonies later during the war, but the situation was never deemed stable enough for the successor of the Dutch crown to settle there permanently.^(6)
While we wait for a quality answer that follows the rules, there is a slightly similar question about the British Royal Family's plans and preparations to move to Canada during WWII over here.
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