I personally believe it bothers on the individual's objective and interests. If the objective of the writer is to educate or inform his readers on a particular subject matter of interest to him, then such a person may choose to stay focused on one TOPIC or SUBJECT of his/her expertise.
If on the other hand, one just feels like hubbing for a hobby then in that case one may choose to write on any topic of interest at any time. Thereby resulting in one writing on very different topics always.
The bottom line however is that none can be said to be better except that the one who writes on one topic becomes an authority in that topic and gets consistent readers and followers who have an interest in that topic.