Scaling is mostly to blame here. At the start of
Uldir, Blizzard was quick to give Beast Mastery a 5% nerf. As the weeks wore on and everyone got gear, Beast Mastery slowly sunk down the charts. The 5% nerf was premature.
There were improvements made to BM scaling in BFA like having pets benefit from increased weapon damage, but there are other factors. Marksmanship simply scales better with weapon upgrades (and in turn, with the Mastery stat on top of it). Weapon damage is worth nearly twice as much for Marksmanship, and secondary stats are also worth more on average (especially Mastery). This is where the scaling issue occurs.
However, the biggest issue may be Azerite Traits. The reason most classes and specs are outscaling BM is because they have more powerful traits, and with all these extra class traits on gear, it just widens the gap that was already there in
Uldir.
