Probably because it's newer.

>>12677
The only right answer.
Though I find TF graphics and gameplay less offensive.

All the trappings around the game I simply prefer and find more interesting (graphics, art direction, 'lore', UI etc.), but gameplay wise the biggest reason is the the character roster and unique abilities each character has. The game offers you a better way to help out your team by playing your character to its strengths which aren't necessary always just shooting dudes, you can be helpful other ways.
I think TF2 is still fine, it's just the natural way of things growing old the next thing coming along that borrows from it, there's just more to Overwatch for me.

>>12679
Finally, a decent answer. Thank you.

OW is newer and has better marketing than TF2. Also waifu character designs.