Because we will already know about it, probably from PAX (they announced Hearthstone at PAX East, just this year) or maybe from SDCC (not as likely). I don't see why so many people are stuck in the 2+ year release schedule when Blizzard have stated many times they want to get patches and expansions out faster. That is the 18 month expansion life schedule and the current rate of patch releases bears it out. 18 months would put the next expansion in late April/early May. In order for that to happen, they would have to have started the beta testing 6+ months before that.
That said, I look for closed beta to start as soon as late September with the open beta floodgates coming at BlizzCon with the announcement of a new Annual Pass. November is FAR to late for them to announce an expansion for WoW and keep to that schedule, and with 5.4 Siege of Ogrimmar hitting the PTR in the next few weeks, we will see it hit live come late August/early September. There will possibly a patch 5.5 that will prep for the next expansion (which I assume from the in game hints to be a return of the Burning Legion).
Keep in mind, I don't work for Blizzard, but my best guess based on what Blizzard has said and what they have done so far this expansion cycle. Keep in mind, they just brought back some of the team that was working on Titan to help out with other projects (including presumably, WoW) so that would seem to indicate that they have the developer resources to easily meet an 18 month release target with current assets.
So if they don't have the Blizzard Xpac to announce, what will there be at Blizzcon? SCII Legacy of the Void, Diablo III expansion, the new direction for 'Titan', and possibly something new and huge (Warcraft IV?).