Why did Black resign this game? This is a game between Jose Raul Capablanca and Viacheslav Ragozin, Moscow 1935:

[FEN ""]

1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.a3 Bxc3+ 5.bxc3 d6 6.Qc2 O-O 7.e4 e5 8.Bd3 c5 9.Ne2 Nc6 10.d5 Ne7? 11.f3 Nd7 12.h4! Nb6 13.g4 f6? 14.Ng3 Kf7 15.g5! Ng8 16.f4! Ke8 17.f5 Qe7 18.Qg2 Kd8 19.Nh5 Kc7 20.gxf6 gxf6 21.Ng7 Bd7 22.h5 Rac8 23.h6 Kb8 24.Rg1 Rf7 25.Rb1 Qf8 26.Be2! Ka8 27.Bh5 Re7 28.Qa2 Qd8 29.Bd2 Na4 30.Qb3 Nb6 31.a4! Rb8 32.a5 Nc8 33.Qa2 Qf8 34.Be3 b6 35.a6 Qd8 36.Kd2 Qf8 37.Rb2 Qd8 38.Qb1 b5 39.cxb5 Nb6 40.Qa2 c4 41.Qa3 Qc7 42.Kc1 Rf8 43.Rbg2 Qb8 44.Qb4 Rd8 45.Rg3 Rf8 46.Ne6! Bxe6 47.dxe6 Rc7 48.Qxd6 Ne7 49.Rd1  1-0


I understand that Black is in a bad spot, being two pawns down and almost the epitome of cramped (not to mention the promotion possibilities for White), but the game doesn't look obviously lost to me yet.  My suggestion would be 49... Nbc8 forcing the White queen back and then 50... Rg8 countering the sting of White's g-rook.  I've played several variations from this point and all of them end up with White keeping the two-pawn advantage, but not a quick win.  Am I missing a certain move by White to refute anything Black could do here?  How did Ragozin know that there was no chance of a draw?

Edit: It has been shown that after 49... Nbc8 50. Qd2 Rg8? 51. Rxg8 Nxg8 White wins at least the Black queen with 52. Qd5+.  However, if White plays 50. Qd2, Black has 50... Qxb5 winning a Pawn and considerably uncramping himself (or so it seems to me):

[FEN "k1n2r2/p1r1n2p/P3Pp1P/1q2pP1B/2p1P3/2P1B3/3Q4/2KR2R1 w - - 0 50"]
Black is down 2 pawns, pieces completely passive, the pawn on e6 is impossible to get rid of, pawn on h6 is also important, creating potential outpost on g7 for rook. White also has two bishops and controls both open files. There is no need for White to prove anything by concrete analysis. Unless Black has some immediate way to get material back - he might as well resign and save energy for next round.