I agree Silver is relatively moderate, but FiveThirtyEight did more or less grow out of DailyKos [1], so it's not a huge stretch to suggest he personally has a clear preference for one or the other side of the American political spectrum. It's not like he also had a blog on the right-wing equivalents of DailyKos, like FreeRepublic or RedState.
[1] For a year or so before he launched fivethirtyeight.com as an independent site, he posted data-driven election analysis, among other things, at http://www.dailykos.com/blog/poblano
It could also be because the Daily Kos' founder also cofounded SB Nation, one of the larger networks of sports blogs that are well regarded in the baseball analytics community, so it's likely that Silver had some connection/comfort with DailyKos.
[1] For a year or so before he launched fivethirtyeight.com as an independent site, he posted data-driven election analysis, among other things, at http://www.dailykos.com/blog/poblano