Why only write about running?

Is there some unspoken rule that when you start a blog you are expected to write about a single subject? I have no issue with that sort of dedication, but when the possibilities of topics could be endless it feels self-limiting to pigeonhole yourself by default. It's a personal bias, but I see enough overspecialization in academia as it is. Having said that, grad students tend to be the ones you find most hyper-focused; I feel well-rounded professors should be writing blogs too. So it goes.