Because sometimes games hit due to timing, design and heaps of other factors. This is why so many games are made and we all say they are shit. It's basically make 100 games and hope something sticks. That's modern gaming
OW 2 left a bad taste in everyone's mouth, Blizzard entertainment ( developers of OW ) reputation has taken a nose-dive since the launch of OW2, on top of it's horrible monitization scheme, people are hungry for better team based game.
It's free, fun, and the MTX are not intrusive. I've not seen a paid outfit I really want to spend money on. At least for the characters I like to play.
Because the TF2/Overwatch formula is fun. If Valve didn't abandon TF2, or Blizzard didn't routinely screw up their own games, those games could still be incredibly successful. I wouldn't be surprised if there are a bunch of other arcade shooters in the works right now after OW's downfall. Marvel Rivals just beat them to the punch.
It is a good game, with compelling characters with good and easily identifiable skillsets from their comic and movie counterparts. It is easy to hook up to and quick to skip between matches that doesn't take to long to finish.