Well understood areas do not have a lot of active research. Research aims exactly to understand things better, and people always try to focus it on areas where there are many things to understand better.
Failure modes in distributed systems are understood reasonably well, but solving those failures is not, and the theoretical primitives are way far from universal at this point. (And yes, hard too, where "hard" means more "generalize badly" than hard to implement, as the later can be solved by reusing libraries.)
The problem is that once you distribute your data into microservices, the distance from well researched, solved ground and unexplored ground that even researchers don't dare go is extremely thin and many developers don't know how to tell the difference.
Failure modes in distributed systems are understood reasonably well, but solving those failures is not, and the theoretical primitives are way far from universal at this point. (And yes, hard too, where "hard" means more "generalize badly" than hard to implement, as the later can be solved by reusing libraries.)
The problem is that once you distribute your data into microservices, the distance from well researched, solved ground and unexplored ground that even researchers don't dare go is extremely thin and many developers don't know how to tell the difference.