Obliterate was nerfed and buffed a ton because Two Handers Obliterated players, and they needed it to be just right. Most of the changes that happened to Obliterate started in WoD, which was the first expansion that Blizzard tried (and failed) to shoehorn us into dual wielding.
Patches and hotfixes
Legion Patch 7.1.0 (2016-10-25): Damage increased by 20%.
Hotfix (2016-09-23): "Obliterate damage increased by 19%."
Legion Patch 7.0.3 (2016-07-19): No longer has a chance to cause your next [Howling Blast] or [Icy Touch] to consume no runes. Damage increased by 170% (35% for each weapon).
Hotfix (2016-03-03): "Now deals 10% less damage against another player."
Warlords of Draenor Patch 6.2.3 (2015-11-17): Damage has been increased by 10%.
Hotfix (2015-06-29): "Obliterate should now correctly deal full damage against other players in PvP combat."
Hotfix (2015-06-25): "
Frost Strike and Obliterate have been reduced to their pre-6.2 damage levels.
Obliterate once again deals full damage to players."
Warlords of Draenor Patch 6.2.0 (2015-06-23): Damage has increased by 20%, but deals only 80% of normal damage against player-controlled targets.
Warlords of Draenor Patch 6.0.2 (2014-10-14):
Now deals 25% more damage on both main and off-hand weapons, but no longer deals additional damage for each disease present on the target.
Damage has been increased by 30%.
Mists of Pandaria Patch 5.4.0 (2013-09-10): Now deals 250% weapon damage (up from 230% weapon damage).
Mists of Pandaria Patch 5.0.4 (2012-08-28): Now a Frost specialization ability. Weapon damage increased from 150% to 230%. Bonus damage removed.
Cataclysm Patch 4.2.0 (2011-06-28): Base damage has been reduced to 150% weapon damage, down from 160%.
Cataclysm Patch 4.0.3a (2010-11-23): No longer consumes diseases.
Wrath of the Lich King Patch 3.0.2 (2008-10-14): Added.
https://wow.gamepedia.com/Obliterate
Meanwhile, Frost Strike was just repeatedly buffed ad nauseum because dual wield-only death knight under performed.