Coming to this a little late. For researchers outside the magic .ac circle, I have access to tons of relevant economics working papers, via IZA, REPEC etc, however if I want to see if there is research relevant to my policy-relevant research in sociology, anthropology or psychology, it may as well not exist, being behind paywalls that my funders won't pay for accessing. The policy-makers I'm writing reports for can't see the papers either so would be required to take any summary on trust, which isn't something I'd want to encourage.

Therefore, and not surprisingly, the economics works get referenced and the others don't.