It's not like I have particular insight but I think it was a combination of a few factors.
1) Opponent: from what I've dug up, Seattle loves to run zone coverage. Keon and Shakir have had no issue beating that this year. Seattle has also ranked near the bottom in run defense.
2) Field conditions: it was a downpour with the field and ball slick (as we saw many times)
Were I, a complete novice, to cook up a situational game plan my immediate thought would be to explore the team strength/weakness by putting primary attack on the run game (Cook went off 100+ yards, 2 TDs) and shorter timing routes. Those are things it takes time to build chemistry on and I don't think Cooper is there yet, especially when you had two options that were working wonders