There are a bunch of reasons why a Pokemon can be good (or totally broken) in one format and totally useless in the other one. Here are some of the more common reasons.
Mechanics that work only in doubles (follow me, helping hand, pollen puff, etc.)
This one's pretty obvious. If a Pokemon is good because of one of these mechanics, it's going to be a lot worse in singles.
Examples: Amoonguss (rage powder), Tatsugiri (commander)
Temporary field effects (screens, tailwind, and trick room)
These moves tend to be a lot more useful in doubles because 2 of your Pokemon can abuse them each turn. This most obviously benefits the setters, but the abusers are also better in this metagame.
Examples: Alolan Ninetales, incarnate Tornadus, Diancie
Intimidate and guard dog
Intimidate is obviously good in singles, but in doubles, it affects both opponents, and both of your Pokemon can get a free turn when an opponent tries to switch out to reset its stats. Intimidate is common enough that guard dog becomes an actual reliable strategy.
Examples: Incineroar, Okidogi
Arena trap
Trapping and checking one opponent is much easier than trapping and checking 2 of them at the same time. Earthquake also does less damage in doubles. Note that shadow tag is still banned in doubles OU, likely because shadow tag Gothitelle has better stats and moves and can check a non-zero amount of threats.
Fast pivots
One common strategy in singles is switching a fast pivot into whichever Pokemon can check your opponent. This strategy is worse in doubles because it's easier to double target a switch-in and more difficult to check 2 opponents at the same time.
Examples: therian Tornadus, Cinderace, Palafin, baton pass, shed tail
Hazards and hazard control (rapid spin, defog, court change, and magic bounce)
Because double battles are shorter and switching is less common, hazards tend to be less useful. Hazard control is consequently also less useful.
Examples: Hisuinian Samurott, Cinderace, Corviknight, Hatterene, Iron Treads
Frail sweepers
Sweeping is easy in singles, as a sweeper can keep itself safe just by outspeeding and knocking out opponents. In doubles, there's one more opponent that could check you and spread moves are weaker, so sweepers often need more support (fake out, follow me, intimidate, etc) in order to work.
Examples: Deoxys, sky Shaymin, Iron Valiant
Walls and defensive pivots
Walling is also easy in singles, as you can switch in a Pokemon that walls all your opponent's attacks and then proceed to enjoy your free turns. If the wall also has a pivoting move, it can take a hit and then safely switch to a frailer teammate. Walling 2 opponents in doubles is much more difficult, especially if one is a physical attacker and the other one is a special attacker.
Examples: Galarian Slowking, Moltres, Gliscor, Alomomola, Corviknight, Pecharunt
There's a few other Pokemon, like Ceruledge and Great Tusk, that are useful in singles and useless in doubles for more complicated reasons.
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