Why do tens of millions of people visit Mexico City every year? What is there to do and see?

exc-5d1f7287345aa60001e521af
exc-5d1f7287345aa60001e521af
Why do tens of millions of people visit Mexico City every year? What is there to do and see?
exc-5d15840c59fb7700015fcd9c
When did leisure travel become what it is today? How did it start?
exc-5bf1b15503ce64bc2309a025