Originally Posted by
Hitei
There are a number of reasons:
•The expansion is being set up as a war to end wars between the factions, so it is unlikely they will just go back to default and working together but hating each other.
•Unlike with MoP's Garrosh/Vol'jin, players are being given choices and the devs have said, with regard to those choices, that they want them to exist where they matter, which creates a divide Horde side between players in contrast to MoP. Essentially making it a lot harder to just shift Sylvanas to villain and reset to the default factions.
•The factions are growing increasingly murky. It's clear that Baine is totally willing to work with the Alliance peacefully (which the Tauren have been doing forever on some level anyway), at the same time, the Night Elves have effectively stepped back from the Alliance. This again, makes clear resolution to the default more difficult.
•Similarly, there are storythreads that appear to be opening the factions to increased mingling, stuff like the Horde-aligned humans in the Horde war campaign, and Calia being an Alliance-aligned potential forsaken leader
•On a gameplay level, there is an increasing sense that the faction barrier for content should just be done away with. People are concerned with the differences in raiding environment, there's longstanding issues in PvP, both in levels of engagement and racial advantage that would be easily dealt with if you could just group with whoever, whenever
•Blizzard has been increasingly relaxing the divide to begin with, with things like Merc mode,