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?
Old generation lower tiers are locked, so Pokemon can't drop from OU to UU in an old generation.
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.