The reason why Warfronts in World of Warcraft are not accessible to raid groups is a trivial one, which the Game Director has now revealed.
Warfronts in World of Warcraft are a game content for 20 players. However, even though that corresponds to a raid, players cannot sign up as a raid for it. A maximum of 5 players can sign up in a group, entire raid groups must stay out.
The reason for this has been answered by Ion Hazzikostas, the Game Director of WoW, in a Developer Q&A.
The balance is meant for random groups: Warfronts were designed from the beginning for “Pick up Groups,” meaning random groups that get to know each other only by signing up for the battle.
For proper, coordinated groups, the whole thing is simply not designed. According to Hazzikostas, the Warfront would be boring because a coordinated raid team would rush through the Warfront faster than the map design can keep up.
This would cause damage dealers to wait at the city gates, having taken everything, while the rest is still farming resources for the siege weapons. This would lead to long waiting times and simply not provide an enjoyable gaming experience.
This guide shows how to successfully navigate through the Warfront.
Heroic Warfronts are an option: However, Hazzikostas does not want to rule out that one can never sign up as a raid for the Warfront. The developers are currently discussing the topic, and perhaps there will eventually be heroic Warfronts that require more coordination or a full raid group. However, this is not planned yet.
Would you wish that Warfronts are accessible for entire raid groups? Or is it good that you can only sign up with small groups?
It was also clarified in the Q&A that allied races will soon be easier to unlock.

