Lack of such promising theories is precisely the point (I guess we are talking specifically about the problem of quantum gravity). Everyone is working with the same 1.5 old approaches and the progress has stalled. Of course individually this strategy is rational because if you try to think of something new and it doesn't work out (which is very probable) your career is toast. But imagine what the brainpower poured into string theory could achieve with a more breadth-first search strategy.
The two most common fields studying it are: 1) Development 2) Labor Although there are a number of Macro-Economists who have studied the impact of inequality on growth. The reason you may not read about it is that the relationship between inequality and growth is a largely "solved" problem. It is "unfashionable" because of that.
I think being unfashionable is more viable in fields that are meritocratic and don't require popular approval to succeed.
Economics: There are surprisingly little people studying very large, grand, topics such an inequality. You've probably heard of the handful of economists that do. It's unfashionable due to the influence of the Chicago school and it's focus on free market principles, as well as many other factors.
Physics: Good luck getting any interest or funding if you are not studying the currently dominant theory in your field (even if there has been no progress in decades).