Why do viability rankings change in doubles and singles?

Question

shouldent the viability rankings of different mons remain largely the same in singles and doubles of a format? the mons available dont change so why does viability change?

for example, in sv doubles both armarouge and ceruledge are UU mons while in singles ceruledge is OU while armarouge is RUBL
why is this so?

Comments

All there really is to be said is singles and doubles are very different. This is an expected outcome.

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what are the ways that they are different in that cause viability of mons to increase or decrease

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Certain moves and abilities are much stronger in doubles compared to singles. For example, Incineroar is a doubles staple because it gets Fake Out + Intimidate + Parting Shot. But that combo isn't very important in singles, so Incineroar is much worse and there'll be no meta forming around good matchups against it etc.

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i guess you could make that an answer

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Fast sweepers like Espathra and incarnate Landorus tend to be a lot more broken in singles because they can't be double targeted.