In Nakamura v. Friedel in 2009, (22. Bc1) why did Friedel resign? After White's 22. Bc1, it seems like responding with Rd2 would keep the game alive for Black. I assume Friedel's a more insightful player than I am. What am I missing? That is, from an experienced player's viewpoint, why do we assume that staying alive with Rd2 is not worth the effort, and how would White be sure to win regardless?

[fen ""]
[White "Nakamura"]
[Black "Friedel"]
[Event "US Championship 2009"]

1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6 4. Ng5 d5 5. exd5 Na5 6. Bb5 c6 7. dxc6 bxc6 8. Bd3 Be7 9. Nc3 O-O 10. O-O Rb8 11. h3 c5 12. b3 Rb4 13. Re1 Bb7 14. Ba3 Rf4 15. g3 Rd4 16. Nf3 Rd3 17. cxd3 Qd3 18. Ne5 Qf5 19. g4 Qf4 20. d4 Rd8 21. Qe2 Rd4 22. Bc1
22...Rd2 saves the queen, but black is still down a massive amount of material. After white takes the rook on d2, he has two rooks for just a bishop. With such a huge material deficit, there is no possible hope of defending with normal play. The only chance for black would be to have an immediate attack on the king or other very strong compensation, which is not the case in the game. White's king is slightly open, but black has no way to take advantage of it and white can easily cover any threats to his king. Since black has no meaningful way to generate any play to make up for the material, he resigned.