I cant also say that Usyk and AJ are close and have equal skill and caliber. But I would say so because they have two completely different fighting styles. The way I see AJ - he more relies on his punching power, he does not make long series of strikes. Usyk is different, he instead likes series of punches. I think without searching in youtube, you wont remember any fight where Usyk has won by knockout that was a single punch or combo of two. AJ has enough 1 punch knockout highlights.
Because he is more of a technical boxer than a brawler. He is also a shorter heavyweight, so he really needs to be smart inside the ring and avoid getting caught clean.
Maybe he is not as popular because of his style, but we cannot deny that he is an undisputed champion. 25 wins with 16 KOs is still a very good record. Usyk is the kind of boxer who can beat almost anyone in the heavyweight division. At this point, he may not have many real challenges left, so retirement might already be close.