Seeing backpackers in Japan is not common as in Europe and usually tourists bring suitcases with wheels from their home countries. Due to platforms are built higher than in Europe, you do not have to climb the steps to get in and get off the train. Also you do not have to pull up the luggage so high to put into the train.
On the other hand, Japanese trains are not equipped with luggage spaces near to the doors. Although limited express trains connecting the airports and the cities have luggage spaces on each side of the cars, Shinkansen does not have any luggage space close to the door. Therefore you have to carry your luggage to your seat and put it on the shell above you.
Leaving luggage near to the door or on the corridor is unauthorized and bothers other passengers. Then, Japan is quite nervous on unattended luggage. It does not mean that someone wants to steal your suitcase but suspicious items.