It's worth noting that LW has a lower barrier to participation and a more community-centric focus, while OB has shifted to be explicitly Hanson's blog; these are clearly pursuing distinctly different goals. The simplest hypothesis is that Hanson and Yudkowsky disagreed on direction and decided to part ways.

And why couldn't that goal have been pursued within a LW-powered OB? It's not as if there is all that much competing content, and surely we can expect further technical additions like tags. Entirely separate sites, with all the implied overhead and loss of network effects, to pursue separate goals only makes sense if the goals are contradictory.