As was already said, some people get sick. Also, many more people prefer to be facing forward on a train. I know I personally do. Sure, I can handle facing backward with no problems, but I just prefer facing forward.
I much prefer facing backwards, because I know in the rare case of crash that I am going to be much safer.
This is also true of aircraft and cars and buses and coaches. Sadly, most people do feel uncomfortable facing backwards and so the risky forwards-facing seating is used.
