Why is 5.c4 playable vs. Sicilian Kan, but hardly seen vs. Sicilian Taimanov? Why is 5.c4 considered playable against the Sicilian Kan, 

i.e. after 1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 a6 5.c4, 

but hardly seen against the Sicilian Taimanov? 

i.e. after 1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 Nc6 5.c4?!
As the knight is already developed on c6, black can immediately create counterplay in the center: 4....Nc6 5.c4 Nf6 6.Nc3 Bb4.

Clearly, 7.Bd3 (the mainline after 4....a6 5.c4 Nf6 6.Nc3 Bb4) is not possible now, as black wins a piece by 7....Nxd4.
Instead, white has to defend the pawn on e4 in another way. Both after 7.f3 and 7.Qd3, black has the strong reply 7....d5, after which white has to be careful to maintain equality.

The best move is probably 7.Nxc6, which has been played by several grandmasters. However, this is already a slight concession by white and black should be fine after both 7....bxc6 (e.g. Kramnik-Kasparov, Karjakin-Grischuk) or 7....dxc6 (e.g. Dubov-Radjabov).

Note that the position often arises via the English opening: 1.c4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.e4.



      [StartPly "7"]

      [FEN ""]
      1.e4 (1.c4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.e4) c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 Nc6 (4...a6 5.c4 Nf6 6.Nc3 Bb4 7.Bd3) 5.c4 Nf6 6.Nc3 Bb4 7.Nxc6 (7.Bd3 Nxd4)(7.f3 d5)(7.Qd3 d5) bxc6 (7...dxc6)