You bring up a good point. Journalists should explore to what extent there are diminishing returns to writing articles on other candidates. Statistics tells us that the most efficient way of exploring this hypothesis is with a multi-armed bandit algorithm. But before I go into that, I think it makes sense to break this problem down into two questions:

(1) Given equally interesting ideas for articles to write on each candidate, which candidate should a journalist write on?

(2) How much investment is required to write an interesting article on each candidate? It might require less work to write something interesting on Trump than on Clinton.

The right way of answering question (1) is with a dynamic multi-armed bandit algorithm. Such an algorithm dynamically explores the problem of diminishing returns. At this point, given the data we have, such an algorithm would suggest you should write on Clinton the vast majority of the time if you're interested in page views per article, and would suggest you write about Sanders if you're interested in bringing in external referrals from facebook and google. If journalists followed the advice of such an algorithm and wrote so many articles on Clinton that readers started to lose interest, then the algorithm would begin to suggest you write on someone else. If there's enough interest in this article, I might write up a follow-up where I fit a model that tells journalists what topic to write on, given that they it's just as easy to write an article on each topic. I could update this model every once in a while to make sure it detects those diminishing returns in time.

Question (2) is more difficult to answer and requires more domain knowledge. I would say it is possible at any moment to write hundreds of interesting articles on each candidate---the real question is how much work it takes. As I mention in the blog post, I am convinced that journalists find it easier to write interesting articles about Trump. So in some sense it's rational for them to do so: the 'return on investment' is higher because it's so cheap to churn out another article on Trump's latest soundbite. However, one could also argue that -- in the name of increased page views, or in the name of a functioning democracy -- they should make the extra effort to write an interesting article on the other candidates.