it is not about a “partisanship”, it is about having a guild where you raid and having arena teammates with whom you enjoy playing arenas and in general having friends on the faction you currently play at. if they are not willing to swap to the other side, why would you want to do it alone? in some cases maybe full guilds will change over but for the most people they will not change sides.
maybe you are a lonely wolf with no friends and no guild, in that case I understand that one would jump to the ally side straight away, but for the majority of players it is not that simple
even blizzard has stated it themselves based on community responses: