Knock Off didn’t need a buff at all, it was very common on balance and stall teams before Gen VI. The buff just made it also good on offence to the point that almost every team carries Knock Off somewhere, which was just bizarre.
I think the three the reasons for Knock Off being used SO much is that
- There's no actually good phisical Dark stab outside of signature moves. The only one that has higher BP (night slash) only has 5 more, and while it has higher crit chance, you don't want to rely on crits to win to begin with, and might as well have the utility and spammability of Knock Off which will often hit for double damage anyway.
- Lot of fighting pokemon want a way to hit Ghost and Psychic types that wall them, and guess what, Knock off is Dark type and hits both supereffectively. It's like, the dream coverage for them.
- Defensive pokemon usually won't be doing much damage anyway, so expecially if they have particularly low offensive stats (hello Toxapex), slapping Knock Off in a 4th moveslot just to add more issues at breaking them is another basically-no-downside choice.
In general, the "no real downside" combined with lack of better options (what are you going to run, night slash / shadow claw?) has a lot to do with its omnipresence, moreso now in gen 8 where it doesn't have to compete with Pursuit.
Gen 6-7 with Megastones and then Z-crystals tecnically indirectly nerfed Knock-Off effectiveness as base Dark phisical stab / coverage on offensive mons that had other options, but gen 8 basically brought back its spammability due to having no non-removable items again.
On other hand, gen 7-8 introduced a couple more powerful Dark type attacks, notably Darkest Lariat and Throat Chop, both formerly incineroar's signature moves.
Throat Chop has a relatively wide distribution but the secondary effect is still far too situational compared to Knock Off's reliability.
Darkest Lariat has a pretty small distribution and again, its side effect is far too situational as well.
On side note, VGC actually has a solid alternative at last (Lash Out) which works very well due to the fact Dynamax is a thing.
Lot of Dynamax moves reduce stats which trigger Lash Out's double damage, and Knock Off also is pretty limited in distribution when tutors are not factored in.