It's a Jewish tradition important to Jewish people apparently and an expression of their identity and non-Jewish people want to stop the practice even though it doesn't affect them and they don't engage in it themselves.
Firstly I disagree that it doesn't affect non Jewish people - indeed I would argue that every single person directly affected by infant circumcision is non Jewish at the time of the operation. Why? Because a new born baby is no more Jewish than they are Christian, Muslim or atheist - they may be the babies of Jewish parents, but they aren't Jewish. They may, or may not choose to become Jewish when they are older, but at 8 days (or similar) they are not Jewish - indeed the cannot be as they simply don't have the cognitive ability to understand, let alone believe in any religious belief or doctrine.
And to suggest that the new born baby of Jewish is de facto Jewish (or Muslim or Christian or atheist, or Conservative, or Republican) as if there is a Jewish gene or a Muslim gene etc runs roughshod over the very fundamental human rights of freedom of religion/belief, in that your religion or belief is a choice, which is the individual's to choose, not to choose or to change.
Secondly the notion that broader society should simply turn a blind eye to what goes on in particular cultural groups because 'it's their culture and it doesn't affect us' is an abrogation of our fundamental duties as a society which include setting basic standards and protecting the most vulnerable in our society, which would necessarily include new born babies. If you take your view then FGM is OK if it is a cultural tradition and it 'doesn't affect the rest of us and we don't engage in it' - likewise other forms of ritualistic child abuse. Why not child sacrifice - provided it 'doesn't affect the rest of us and we don't engage in it'.
No - that's not how our society runs, thankfully. And we are affected by it - as a member of society we are all affected by everything that goes on in the name of that society.