What are the main differences between Graduate school in the UK vs US? I graduated from a university in Canada, and I have been focusing on applying to North American universities even though my GPA isnt the highest.
I recently started searching for MSc programs in the UK, and I noticed that entry requirements are much easier. For example, for the course-based , we don't need to secure a supervisor yet students still work on a research project. Whereas when I tried applying to McGill university in Canada, I was told that not having a supervisor will not allow me admission to the program.

Why is admission to graduate school much lenient in the UK compared to the US/Canada? are schools in North America known to have the best reputation in research? why aren't many people going to study in the UK if things were that easy? 

I'm interested in knowing the main differences between the UK and North American universities.
I don't think your measure of 'easiness' is really correct. Finding a supervisor isn't a measure of how strong a student you are, it's a measure of how the course works and so in what circumstances a supervisor will take you on. 

You haven't said what you mean by 'graduate school' in North America. Usually you would compare doctoral programmes, and there you'll find the UK 'harder' on average, because a US programme begins with taught courses that don't form part of the UK programme (as much).

A taught MSc in the UK is only just above undergraduate. You essentially do a typical undergraduate year of taught courses (which may be very very similar to ones taken by undergraduates) and then spend the summer doing a 3 month research project. You therefore don't need to start with a supervisor, as you have 8 months to find one, and you will always be allocated someone. Just below this are a selection of 'undergraduate masters' courses, where undergraduates study for a fourth year, beyond the normal degree programme. They basically do the same as the MSc except for missing the final project (or doing one instead of one of the taught modules).

If you looked at an MSc by research, I doubt you could get on to that without a supervisor. That is closer to doctoral study. Slightly up from that is an MPhil, which is roughly the first part of a PhD, and actually is in some subjects.