I'm about to read Mazur's paper "Modular curves and the Eisenstein ideal". It's so long and difficult for me, but I found a website which shows the Mazur's theorem. This is very short and very very easy comparing with the original paper.
Why is the original paper very long and difficult? Or equivalently, why is this website short and easy?
The original paper needs many advanced theory, for example, Katz's modular forms. But the website does not use this theory. And I think that there are more advanced theories which are used in the original paper, but are not in the website. What's happening?