For starters, Fell Stinger is a Bug type move with 50 Base Power. That's not a lot. It does have that amazing three attack stat boost, but it's just not powerful enough. When you have a max moveset of four moves, most (if not all) Bug type Pokemon will have one attacking Bug type move. X-Scissor for Physical Attackers and Bug Buzz for Special Attackers are common. X-Scissor and Bug Buzz have powers of 80 and 90 respectively, making them relatively good moves.
It also has very little distribution between Pokemon. This doesn't have to do with why it's "usually useless" but why you don't see it in competitive play. In Gen 8, the only Pokemon that can learn it by level up are Qwilfish, Vespiquen, Drapion, Escavalier, Genesect, Togedemaru, Naganedel, and Buzzwole. That's.... Not very many. Especially when 3 are Legendary/Mythical. Many of these Pokemon have much better ways to utilize their powers then Fell Stinger.
All in all, Fell Stinger just ain't that much of a good move. Drapion uses it sometimes, and if your playing Little Cup with Skorupi, then definitely you will want to use it but other than that, it's pretty useless.
Hope I helped
Source