Is the the remnant somach attached to the intestine leading to the new point at which the pyloric valve meets the intestine?
This part of the intestine, which now carries no food, is the bile path. Perhaps there are other secretions of the remnant stomach which are potentially beneficial.
Or, it could be just simpler to leave it there. The stomach remnant is vascularized, and, if you remove it, you have to tie off all those conduits. This takes time, and duration of surgery is best minimized. That said, I suppose you still have to staple the remnant, and that too takes time
Just guessing, I have no idea of the reason