With every new pokemon gen that comes out, they introduce more and more new pokemon, new moves, new abilities, new held items, and new forms to deal with in competitive battles. Drastically changing the preexisting tierlists a lot in the process with each new Pokemon game.
Despite being just an "OU banlist", the Uber tier has proven to be the most consistent tier out of all of them. You can see this with how little the Smogon Uber tierlists have changed over the past seven gens. Retaining most of the usual mainstays from the previous gen's Uber tierlist with each new, passing generation.
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First gen; no returning ubers, because there is no previous gen.
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Returning pokemon from gen 1: Mew, Mewtwo
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Returning from gen 2: Ho-Oh, Lugia, Mew, Mewtwo
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Returning from gen 3: Deoxys (all 4 forms), Groudon, Ho-Oh, Kyogre, Latias, Latios, Lugia, Mew, Mewtwo, Rayquaza, Wobbuffet, Wynaut
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Returning from gen 4: Arceus (all types), Darkrai, Deoxys (all 4 forms), Dialga, Giratina (both forms), Groudon, Ho-Oh, Kyogre, Lugia, Manaphy, Mewtwo, Palkia, Rayquaza, Shaymin-Sky
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Returning from gen 5: Arceus (all types), Blaziken, Darkrai, Deoxys (all 4 forms), Dialga, Genesect, Giratina (both forms), Groudon, Ho-Oh, Kyogre, Kyurem-White, Landorus, Lugia, Mewtwo, Palkia, Rayquaza, Reshiram, Shaymin-Sky, Zekrom
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Returning from gen 6: Aegislash (both forms), Arceus (all types), Blaziken (both forms), Darkrai, Deoxys (all 4 forms), Dialga, Genesect, Gengar-Mega, Giratina (both forms), Groudon (both forms), Ho-Oh, Kangaskhan-Mega, Kyogre (both forms), Kyurem-White, Landorus, Lucario-Mega, Lugia, Mewtwo (all 3 forms), Palkia, Rayquaza, Reshiram, Salamence-Mega, Shaymin-Sky, Xerneas, Yveltal, Zekrom
As you can see, whenever newer uber pokemon are added with every revision to the latest Uber tierlist, the new pokemon feel more like Additions rather than Replacements. This is largely because uber-class Pokemon are so powerful, it's generally hard for the uber mainstays like Arceus to "fall down a tier". Making the Uber tierlists the relatively Most consistent between the different generations. Especially when you compare it to lower tiers like OU, where the tierlists can rapidly change because new threats can Replace the old ones. Or even knock them down a tier, if old pokes are made unviable in the latest OU metagame.