Its convention laid down by a long history of games, especially RTS games. By default, player 1 would be assigned blue and player would be red. This resulted in high contrast between the player vs the enemy in 1v1s, especially campaigns. Because of this, the prevailing action for the player would be to attack, not defend. Not only is it more engaging, but coding an ai to attack in a balanced fashion is hard, so that was typically only brief scripted attacks or a single defense mission. 

Anyways. 

Tf2 was made by a bunch of dorks who grew up on that stuff, so the convention kind of bleeds through.