Join Date: Dec 2004
Location: Riding the rails
Programs: Japan Forum, Skyteam Elite Plus, BW Diamond Select, HHonors Gold, NWA, DL, NH
Posts: 1,936
It is because Nikko is on a branch line.
If you want a guaranteed seat and don't mind paying exta they run direct all reserved seating trains at certain times as well from both Shinjuku Station (in joint operation with JR) and Tobu Asakusa Station, slightly faster and more comfortable.