Why did the BNA colonies unite?

In the 1860’s both upper and lower Canada the economy wasn’t very good. But with the help of the railroads people thought that it would help the economy but it didnt. This was one of the reason that they united because the railroads helped people get around faster, but since it was 2 seperate colonies no train ran through both parts of Canada. The reciprocity treaty influenced the 2 colonies to unite because it was meant to help lower Canada by uniting with the states but why wouldn’t they just unite ALL of Canada with America. The corn laws made it that Canada’s economy would go down because there would be less profit in grain. So this is when they decided to unite canada ans make it one country so upper and lower could help each other out with their economy issues.