Why would Óðinn arrange things like this?
Because he is engaged in a game of chess against the vast, devouring nothing and if all the pieces aren’t in play precisely where and when he needs them to be the consequences will be most dire.
And he’s even willing to let the enemy take out one of his most important pieces, if it gains him a strategic advantage.
Which is how Óðr was for a time lost to the Tívar, and mostly to history’s pages as well.
He never would have sent him on that final quest, knowing what lay ahead for him and all of them, had it not been absolutely necessary.