There is a whole book about it: http://wdp.wharton.upenn.edu/b...
TL;DR: HR managers are way too picky and specific in their requirements. They only want to hire people who are currently doing the exact same job. They increasingly expect people to be willing to commit to shorter and shorter contracts for tasks that should take far longer to do right. But primarily: HR managers have, as a group, turned into power-mad, elitist, snobs who routinely throw away resumes after barely a glance if they feel like they just wouldn't like the candidate; just because they can. The personal bias being applied here is enormous.
The worst part is that the actual hiring managers are desperate to get the role filled, and would have been happy with half of the people the HR manager rejected. But the HR manager is using their position to attempt to control the durection of the company, or just fill it with "their kind of people."
So, do everything you can to bypass HR.