Most of them probably hadn’t hit the corner under the rapidly worsening conditions.
All the cars had been by within the yellow during those worsening conditions. I watched the entire race, they started with the safety car on the track, under wetter conditions. It's very unfortunate. The area directly preceding the crash site was under a double-yellow flag, which means to slow down, and be prepared to stop, and that no passing is allowed.
The area immediately adjacent to the wreck did go green about 15-20 seconds before Jules hit the tractor, and I've not seen replays of flags in the sections just prior to the recovery area, nor have I seen any other angles of the crash (overhead would have been best), except the one. In the video I've seen, it appears there is a camera closer to action near the safety barrier that may have followed him all the way around the corner and into the tractor. It's a nasty accident, but all the drivers were out there under the same conditions.
I think, if anything, the tower flagger and lights right in that area should have stayed with the double yellow flag until the equipment completely cleared the area.