Why is 15.cxd6 better than 15.cxb6 here? In Radjabov vs. Gelfand (2013):

[FEN ""]
1.Nf3 c5 2.c4 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.g3 Qb6 7.Nb3 Ne5 8.e4 Bb4 9.Qe2 d6 10.f4 Nc6 11.Be3 Bxc3+ 12.bxc3 Qc7 13.Bg2 e5 14.c5 b6 15.cxd6 Qxd6 16.O-O O-O 17.f5 Rd8 18.Rfd1 Qa3 19.Rxd8+ Nxd8 20.Bg5 Ba6 21.Qd2 Qe7 22.Rd1 Nb7 23.Bf3 Rd8 24.Qc1 Rxd1+ 25.Bxd1 Nd6 26.Bxf6 gxf6 27.Qe3 Qc7 28.a4 Qd7 29.a5 Nxe4 30.Bc2 Qb5 31.Qf3 Ng5 32.Qg2 bxa5 0-1


On move 15, White plays 15.cxd6 instead of 15.cxb6. Why? I can see two reasons to prefer the latter:


It preserves a weak pawn on d6 (and by extension a weak square on d5).
It makes a new isolated pawn on b6 that's on a semi-open file too.


It seems to me that the obvious plan is 15.cxb6 intending O-O, Rfd1 and Rab1, etc. However, Radjabov didn't play this, and Stockfish doesn't like it either; at depth 24 it says 15.cxd6 is 0.00, but 15.cxb6 is -0.80. What's wrong with taking on b6?
There are a few ways White is at a disadvantage here that I found:


Black’s dominance in the center.
The active black knights.
White is left with 3 pawn islands, which is never considered good.


Look at White’s pieces here-no really good moves are available. The knight can't move up and come into play. The rooks are undeveloped/have no open files to dominate. The bishops don't have room to move freely, and the same goes for the queen.

    [Event "?"]
    [Site "?"]
    [Date "?"]
    [Round "?"]
    [White "?"]
    [Black "?"]
    [Result "?"]

    [FEN ""]
        1.Nf3 c5 2.c4 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.g3 Qb6 7.Nb3 Ne5 8.e4 Bb4 9.Qe2 d6 10.f4 Nc6 11.Be3 Bxc3+ 12.bxc3 Qc7 13.Bg2 e5 14.c5 b6 15. cxb6 axb6 16. O-O O-O


If you continue with this all hell breaks loose for White: 

[Event "?"]
[Site "?"]
[Date "?"]
[Round "?"]
[White "?"]
[Black "?"]
[Result "?"]

[FEN ""]
1.Nf3 c5 2.c4 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.g3 Qb6 7.Nb3 Ne5 8.e4 Bb4 9.Qe2 d6 10.f4 Nc6 11.Be3 Bxc3+ 12.bxc3 Qc7 13.Bg2 e5 14.c5 b6 15. cxb6 axb6 16. O-O Ba6 17. c4 Nd4


Bxc4 is always coming, Ng4 is possible at some point, and Nc2 is a possibility. You can always bring the Queen to c2 to force an exchange of material. White's light squared bishop is still not helping much. Don't forget about the dominating  center pawns. If you play on you'll definitely find Black up by material.  

I will not say "always take towards center,” but it does help you most of the time.