[evanjrowley]

If I was a backend developer (and I am not one FWIW), I imagine I can't point at the back end of production systems that I've worked on because the internals are covered by an NDA, some legal intellectual property restriction, or I'm barred from doing it under some information security policy.

I.e., it may be impossible to create a portfolio for the majority of things you work on as a backend developer. Front-end developers work a lot on public-facing things that lack any of these restrictions, and so it is likely much easier to create a portfolio website. Screenshots of the back-end may land someone in court.