The reason everyone is using Genesect is because everyone always had used Genesect just like Fizz said because of its usefulness.. Every game in which Genesect has been in, it has been an Uber, so if course it is good. What makes Genesect so good is that it has a bunch of options as well as a great Defensive typing in Bug/Steel. Genesect can run both sides of the spectrum effectively with Iron Head, U-Turn, Extreme Speed, Blaze Kick, Gunk Shot and Shift Gear to pair with it On the Physical Side with 120 base Attack. On the Special side, with 120 base Special attack, there is Dark Pulse, Ice Beam, Bug Buzz, Flash Cannon, Flamethrower, Techno Blast, and a whole bunch of coverage moves to help it. Additionally, it can run all different items in a Drive, Choice Band/Specs, Choice Scarf, among other items and still be effective.
So, a combination of these and the fact that you can encounter all types of spreads for it.r. It also boasts a usable 99 base speed that can be boosted by Shift Gear to boost it.
Additionally, Genesect got nothing extraordinarily new in Crowned Tundra to boost it usage significantly, it has just always been used that much. The only noteworthy new move it got was Leech Life, but that isn't great.