Another answer to consider:
Everything this 'mon does is exploitable. Grassy Terrain is cool, but in a screens meta it serves as a double edged sword, and often benefits the opponent more than its teammates. We're also seeing a ton of rocky helmets recently, which punishes pivot spamming (not to mention Rilla often just kills itself with wood hammer recoil into helmet). It hates facing common defensive staples like Pech, G-Weez, Hydrapple, or any of the birds. Offensive teams also have plenty of 'mons that can check it and exert pressure back (Zama, 'Nite, Kyurem, Moth, etc.).
All its sets have very exploitable downsides, which makes it hard to teambuild around.
- Band is heavily prediction reliant and serves as easy setup fodder even when you do click the correct move.
- SD Life Orb sets are powerful, but only ever get one chance for setup, and are easily answered by common anti-offense options due to limited coverage. You can run Tera Blast for this, but then the sets become extremely tera reliant.
- Terrain Extender enables some neat synergy, but gterrain teams are extremely mediocre and rarely worth the teambuilding restrictions.
- Boots helps with longevity problems, but you really feel the lack of power, and it needs specific support to work.
- Scarf exists only for the surprise factor, and suffers from the same damage issues Boots does. Once it's revealed it becomes even easier to play around than band.
It is not surprising at all that Rilla has by far the lowest winrate of any OU mon when you consider this SPL and last SCL. It was brought to 13 games this SPL and managed to only win one of them (7% winrate), with a 33% winrate last SCL over 18 games to boot. The meta is just not favorable to it at all right now.