The question isn't 'why don't academic publish in open access journals', its really 'why aren't the open access journals as good'?
I publish in both. My papers get read *more* in the open access journals. But the quality of scholarship of those papers, and the ones published along with it is lower. The simple reason is that the open access journals need my money to stay afloat - the fees I pay them to 'publish' (really for the copyediting, layout and coordinating peer review) bias their decision making. Its like an investment bank paying a credit rating firm to rate their product. Guess what - that shit smells like roses! Traditional journals are more likely to be hard-nosed, rejecting more papers up front and picking tough peer reviewers. They make their money by keeping their subscribers who want a monthly issue full of rigorously reviewed worked.
So, like my peers, I trust papers published in the traditional journals more than the open access journals, because I know that there is less financial bias. And when I have a choice because I have a great piece of scholarship to publish, I want it in with the other work that I trust myself.