But why are so many mons not used in UU as much as they are in their home tier if they have equal or greater viability in UU than they do in their home tiers?
I think I was able to decipher this, so I'll take a stab at it. I'll use Decidueye as an example, because there's a tie-in to the current suspect test.
Right now Decidueye is one of the better checks to Breloom the UU metagame has access to right now, which is doing a lot of the heavy lifting for its viability in UnderUsed. Its decent special bulk and access to Roost give it a few opportunities to come in on other notable threats in the current metagame (like Mega Manectric, Mega Slowbro, kinda sorta Latias, etc), heal up, and either threaten to trap them or pick them off with Sucker Punch. It's not absurdly useless, but it's not fantastic either. I don't know a lot about the inner workings of the RU metagame, but there appears to be an increased presence of Fire- and Normal-types in the S through B+ rankings. Having Mandibuzz and Shaymin as two of the best Pokemon in the metagame probably doesn't help Decidueye's case either: it can still perform the same roles as it does in UU, but it won't do so as well because of the increased population of Pokemon Decidueye
doesn't beat. There isn't exactly a one-to-one correlation between viability and usage: metagame trends and dynamics, something MrAldo touched on, provide Decidueye with a slightly more favourable metagame environment in UU than what it has in RU. Because Decidueye accomplishes a niche (checking Breloom better than most things) in UU, it's warranted a rank. The significance of that niche determines the rank.
Instead of focusing strictly on the letter grade of a Pokemon's viability, consider the collective of Pokemon in that tier. The VR isn't necessarily an "x is better than y" list, but a more relative ranking of how things perform in their metagame. The ranks are determined by offensive/defensive presence, what role they play in the metagame, and how much support is required for that Pokemon to fulfill its role. The RUVR does its community a disservice (imo) by not including some expanded form of a definition for what the Viability Rankings are. I've provided you the one from one of the original VR threads, as I still find these guidelines to be the gold standard for how rankings are established.
From FlareBlitz's original Viability Rankings guidelines said:
S Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame with little support, and Pokemon who can support other Pokemon with very little opportunity cost ("free turns"). Also the home of Pokemon who can easily perform multiple roles effectively, increasing their versatility and unpredictability. If the Pokemon in this rank have any flaws, those flaws are thoroughly mitigated by their substantial strengths. If there are suspects, they will come from this rank.
A Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame, but require some support or have some flaws that prevents them from doing this consistently. Supporting Pokemon in this rank may give opponents free turns or cannot create free turns easily themselves, but can still do their job most of the time.
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outclassed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
C Rank: Reserved for Pokemon who can be effective given the right support, but either have crippling flaws that prevent them from consistently executing their strategy or are typically inferior to the Pokemon in the above ranks.
Keep in mind that the +/=/- rankings within a letter subset are just relative to those letter-ranked Pokemon. B+ and B- ranked Pokemon are still B Rank Pokemon, one's just slightly more viable than the other.
