Why is not possible to book the train from Pingyao to Datong?

Hi, all the trains from Pingyao to Datong are pass-by trains, which means the trains’ starting stations are not Pingyao. It is said that only the train tickets whose departure stations are also the trains’ starting stations could be arranged online. Thus, you would better buy the tickets Pingyao-Datong at train station by yourslef. For tickets on Shanghai-Hangzhou and Xian–Pingyao routes, they could be booked online.