While designers/bloggers tend to cite scientific studies advocating sans over serif (or the other way around depending on the context), no conclusive scientific evidence to prefer one or the other [1]
However, the historical reason why sans-serif are generally recommended over a serif on a computer screen was a matter of practicality. The computer monitor simply couldn't replicate the tiny detail required by serifs. Higher resolution screens are becoming more common place, which is why you'll see more serifs in use online (IMO... correlation vs causation and all that). Somewhere along the line someone cited scientific articles to help justify this practice, but I'm convinced it became common place was really for practical reasons/aesthetics more than anything.
[1] I haven't personally done a literature review, but this post is great in discussing the conclusiveness around the research (and readability vs legibility vs....) http://alexpoole.info/blog/which-are-more-legible-serif-or-s...
