[StartPly "10"] [fen ""] 1.g4 e5 2.Bg2 Bc5 3.e3 d6 4.d4 exd4 5.exd4 Bb6 *
In the Grob attack opening database posted in this answer the suggested next move is 6.Nc3. However, in most variations of Grob opening I looked at so far, c4 is played by White very early in the opening phase. In the above position I would pick it as the next move also.
Why is 6.Nc3 better for White here than 6.c4?