UML: My Part in Its Downfall

Two problems, first a state machine, or collection of, will be able to describe the behaviour desired much of the time, if you really understand your problem and how to solve it. Agile often doesn't really know exactly the problem it's trying to solve, so that's a problem. If you have the state machines the code is trivial, but it's a lot of work to define to that granularity.

Secondly, system and subsystem decomposition is an architectural skill that is near black art to do well, and requires significant experience to do well, especially on large systems, but once again, done well makes the following work easier.

So having to do big work up front is not move fast and break things, so it hasn't really been popular, note the sort of people really interested are those used to big work up front, eg aviation.