I wouldn't think that UML is unpopular, so much as it's not as helpful as other languages. UML is especially useful in OOP, which Go is not, at least not specifically. And I also think UML can tend to push you into certain paradigms of thinking or design patterns, whereas maybe Go tends to have more flexibility and more tricks available to it that don't lend themselves well to a UML diagram.
To be fair, it's probably been 10 years since I touched UML, so my memory is probably rusty, but I remember being quite into it when designing OOP systems, as it really helped map out the various entities in the system, assign responsibilities and define relationships. It was an excellent way to help design a system that was too big to fit in your head all at one time. But I think I remember that it was pretty object or class focused. Though that could have just been the way I used it.