A lot of the appeal of Hyjal is the fact that it is closed off and inaccessible. That's what makes exploring incomplete areas so amazing; you're making your way into an area that otherwise most people can't get into nor really want to bother with, so it just makes your exploration that much more enjoyable and a bigger sense of accomplishment.
If Hyjal was suddenly opened on all four sides with a red carpet fully laid out begging to be explored by all 9,000+ players, then it would lose a lot of its appeal as it would become just another empty region that players run around mindlessly in. Think the Barrens but with no mobs, NPCs, or quest hubs. Sure people would get excited it at first, but that excitement would quickly dwindle as they discover running around a barren region devoid of any and all content is boring and will eventually go back to ganking and griefing players at your typical PvP quest hubs (Redridge, Ashenvale, Stranglethorn, etc.)
Besides, exploration in inaccessible areas is only permitted as long as it doesn't provide an advantage to either PvE or PvP encounters. If Hyjal was opened up to every single Nostalrius player, people will inevitably start mining it for exploits and the staff would then have to deal with tickets about players abusing exploits or bugs found in normally inaccessible/incomplete areas of the game. I think the server has enough problems to deal with right now let alone opening up inaccessible/incomplete regions to the entire player base to potentially exploit.
