Because like most people I browse my phone while laying down on the couch or doing other things. I dont want to play games with the browser setting, If I wanted horizontal I would allow my phone the option of going horizontal. Setting a site to all doesnt allow this to lock on portrait.
Its really annoying that I wont even go on my own forum because I cant browse it properly when laying down. I dont just stay on my forum from 9-5 at a desk. For the sake of community interaction I think it would be nice to have the feature to lock it to portrait or give us a way to do that