I'm not a great expert on the Grand Prix Attack and its subtleties, so I'm afraid I'd need a few hours to understand exactly what reasons lead White to leave Black's bishop on d7. Even without in-depth analysis, I will give a clue in the following moves as to what I believe to be the reason.
1. e2e4  1... c7c5  2. b1c3  2... d7d6  3. f2f4  3... g7g6  4. g1f3  4... f8g7  5. f1b5 This move begins a more subtle way of playing the Grand Prix Attack, one of the most dangerous lines of the Sicilian. 5... c8d7 Clearly the best way to block the check.