Loyalists who lived in the 13 colonies fled to Canada because Canada was part of the British Empire. In Canada they could still be British. If they stayed in the colonies they would be traitors to the King.
When their cause was defeated, about 15% of the Loyalists (65,000–70,000 people) fled to other parts of the British Empire, to Britain itself, or to British North America (now Canada). Wikipedia
There was much more land available in Canada than in Britain. As @reirab points out,
The entirety of Great Britain fits inside the province of British Columbia alone... 4 and a half times! Granted, most of the land in Canada is also much colder than Britain for a large portion of the year.
Land was very important; land was wealth. Land was more important than income. Land was vital for economic and political rights - the true importance of owning land is difficult to explain without a long article on the theory of British rights & politics.
Simplified since OP said that the Wikipedia article presented linguistic challenges