Why, Because if the truck stops then takes off it will take longer to get across the crossing as it will be at walking pace or less so in this case the truck could check, see the line clear, start entering the crossing and have a train appear when it is fouling the crossing with the inevitable result. I have almost got caught by this as I was crossing a 3 line crossing in the suburbs and moving off when the bells and lights stopped and the boom gates had lifted with no trains coming my restored 1955 truck in Melbourne Peak Hour when there was twice the space I needed on the other side of the crossing. As I crossed vehicles in the lane beside me went past and changed into my lane to get the extra car lengths in the traffic taking the gap I was about to fill. I couldn't go back as the traffic behind me had taken the gap I left. I stopped with the tail of my truck over the fouling line when the boom gates went down. Luckily I was not over the rail and the train wasn't on the closest track to me. All it needed was one or two of the cars to have had a trailer and I would have been in trouble. Note Everything I had done was safe and legal it was the unthinking actions of others that caused the issue.
There was a freight train hit a road train in the northern territory where the ATSB found that at the speed the train was going it would have been over 2.5km away over a hill and round a bend when the road train started crossing the railway line. In other words the truck driver did everything right but was caught out by circumstances. This doesn't mean the train driver was wrong, if anything the engineers that designed and built the crossing stuffed up