While a parent selector would be extremely useful, it has some performance issues that are apparently hard to solve with rendering engines. It's not been left out due to some kind of oversight by the W3C, it's been debated quite a bit at WG meetings. Much the same as element-level media queries (width of parent element).
I think selectors (and combinators and pseudos) in general have become pretty powerful. eg. I have a Selenium test for table sorting which just uses the nth-child selector. It's certainly better than the days when you could only really use element, class or ID selectors.