Why is Dugtrio in OU in Generation 5?

Question

After Arena Trap was banned in Gen 5, Dugtrio's usage plummeted. It is completely unviable, and is utterly outclassed by Excadrill. It's smogon page also talks about how useless it is in OU. However, it states that it is OU instead of UUBL or a lower tier. Why?

Comments

its speedy and excadrill's defensive stats are basically all hopeless and it can kill it with earthquake, plus its good at sweeping when everything it beats is weakened although its paper thin and 100 attack is kinda mediocre

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80 attack actually. If it "not worth using" as smogon says, and Excadrill does what Dugtrio does "insurmountably better", why is it OU?

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This site doesn't have many Smogon users. You're more likely to get a good answer by asking on a site with more Smogon users, such as Smogon. https://www.smogon.com/forums/threads/ask-a-simple-question-get-a-simple-answer-mark-ii-roa-edition.3468567/

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Thanks sumwun

Answer 1

Old generation lower tiers are locked, so Pokemon can't drop from OU to UU in an old generation.
source
Arena trap was banned after gen 5 UU became an old generation, so Dugtrio can't drop to gen 5 UU after arena trap was banned.
source