It's because most of the who's hiring on HN posts are small 2-3 people startups that desperately need a new programmer.. yesterday. They're not a team that can afford to bring someone up to speed, even if that person is amazing in programming other things. Which is really unfortunate.
It isn't just the new startups. Hiring managers believe that the best indication you know how to do what they need done is to have done it before. And their vision of "done it before" is unfortunately very narrow.
It's unfortunate and also a terrible business model. In those cases it's clear the business isn't mature or ready enough to actually hire human beings, in my opinion. Not that I'd agree with their conclusion about who they need to hire anyway. There isn't an experienced developer out there who can't pick up the frameworks and languages (exceptions exist of course, e.g. for quirky languages like brainfuck or paradigm shifts like procedural to functional) these CRUD-by-other-names shops use within a week, and if their business is that close to failure without developer help they can't afford to be picky.
