I suppose that’s because parliamentary parties are much stricter with their membership. A small difference of opinion could lead to the expulsion of a member. US parties can’t really do that, so instead we have caucuses within the parties that vote along party lines most of the time, but differently on some important issues. In a parliamentary system, the caucus members would be expelled and would have to form their own party to have their views represented.
Canada has a first past the post system and we have several parties. This is a uniquely American issue.
I suppose that’s because parliamentary parties are much stricter with their membership. A small difference of opinion could lead to the expulsion of a member. US parties can’t really do that, so instead we have caucuses within the parties that vote along party lines most of the time, but differently on some important issues. In a parliamentary system, the caucus members would be expelled and would have to form their own party to have their views represented.