Why not work on a show that actually needs to be subbed?
Because it's very uncommon to work on a show you didn't enjoy. As such, the less popular a show is, the less people watched it, and thus, the number of people who watched it, enjoyed it, and have the ability to work on it goes down.
Show that need to be subbed are shows that are not popular. Thus, as stated above, there will be far fewer people interested in working on it. That is the reasoning.

In conclusion, the fact that unpopular shows have very few people working on it completely logical. Q.E.D.