Its because there is constant news from Sony with a stream of games ect announced or launching.
Xbox One has game droughts, esp this time of the year (or it just seems that way).
Nintendo? well tons and tons of NX threads.
I agree that there are a ton of NX threats (to which i've also contributed a couple, hehe). Alot of fluff Nintendo posts are primarily trying to brainstorm why Nintendo isnt dominating or should be dominating (which has been going on since the mid 2000's from my recollection growing up online). Also, they try to figure out what went wrong with a Nintendo Direct or some other event. They also have the normal review and discussion topics about games that everyone else has. Nintendo fans are definitely the most invested in the culture of Nintendo as far as Nintendo is concerned.
Most of my friends who play Xbox are every day people who like social competition, so if they arent out and about they are doing something or at home playing multiplayer. Sony's got the dominance part covered because they have games coming in year round if people are paying attention to all types of games and not just AAA western games.