My guess is that the GI is suggesting Remicade first since it can start working faster, whereas 6MP takes a few months. Another thing I've heard of doctors doing is starting Remicade and 6MP at the same time, and then weaning off Remicade to see if 6MP will maintain remission.
(For what it's worth, I tried 6MP first but it's side effects landed me in the hospital just after 3 doses, and I immediately started Remicade)
Female, Age 20
9 Asacol, Rowasa, 1000 mg Canasa, Proctofoam, Rifaximin 2/day
Digestive Advantage (Crohn's & Colitis), 1 Florastor, 50 mg 6MP, 1 Primadophilus reuteri, Remicade (7 infusions), 2.4 g Lialda, Forvia, 6 Colazal, 1 Anucort, Culturelle, Fish Oil, Calcium/Vit D supplement, 20 mg Prozac, VSL #3 DS, Turmeric