I am asked all the time about which faculty positions are hardest to fill. Although the answer is debatable, I think that finding solid Old Testament professors is a significant challenge.
There are a variety of reasons for this. One is that there are just fewer of them. In the Reformed world especially, we have many more folks in systematics, historical theology, and church history. But fewer in biblical studies (especially OT).
Beyond sheer numbers, there may be an even bigger reason that solid OT guys are rare. Arguably, the OT is filled with proportionally more historical and theological challenges. Such challenges include the days of Genesis, the question of evolution, the documentary hypothesis, Mosaic authorship of the Pentateuch, the authorship/unity of Isaiah, the Canaanite conquest, and much more.
In short, OT studies has a lot of potential pitfalls.