Tournament rules for "One for All".

What prevented him from immediately choosing a stronger player for the second slot? I think this is more of an exception, as reasonable captains should prioritize selecting stronger players.

Perhaps because he was voted for and, unwillingly, became the captain. And, lacking complete and objective information, he chose someone he knew.

From a player's perspective, when you sign up for a tournament where player 1 plays against player 1, and so on, you end up in a group where all your opponents are stronger than you, and you become a drag on your team. From a moral standpoint, such a tournament wouldn't be enjoyable.
It's one thing when you play for yourself in a group against stronger players, and another when you lose and let your team down.