Question: What made room 1408 so special? That it's numbers added up to 13, and it was on the 13th floor? Wouldn't that also make room 1426 evil as well? Or was the room just inherently evil?
Answer: It's possible there is no 1426. The rooms may only go up to 1410 or 1415.
Answer: Like many of King's work, it's never explained why the room is evil, it's just something that resides in the room that is evil. The fact that it's on the 13th floor and 1408 adds up to 13 just makes it worse, superstitiously, so perhaps that's why the evilness chose that room. But the 13's don't really contribute to the events inside the room, so other rooms aren't affected.
Bishop73