As things stood, Southshore was safe for the moment, and there was no need yet to worry about the orcs taking it. However, the Allied Army inside Southshore could not defeat the orc forces besieging the town either.
Alterac Valley was the Kingdom of Alterac's sole remaining line of defense. If Alterac Valley fell, the kingdom would inevitably be destroyed.
Moreover, Alterac Valley was the main defensive line preventing the orcs from taking a shortcut north and directly attacking the Alliance's most important city: Lordaeron, the capital of the Kingdom of Lordaeron.
For those two reasons, the Alliance had to hold Alterac Valley at all costs. Those same reasons made Alterac Valley even more important than Southshore among the three main battlegrounds.
Once Lordaeron City fell, the Second War could essentially be declared over. It would mean that the Alliance had finally suffered a complete defeat in the Second War.
Without a leader, even if the magic city-state of Dalaran and the high elves' Kingdom of Quel'Thalas retained relatively intact forces, those two alone would ultimately be powerless to save the Alliance.
Compared with Alterac Valley and Southshore, Tarren Mill was less important. As a result, it became the weakest link in the Allied Army's defensive line against the Orcs Main Force's northward advance.
In fact, ever since this defensive line had been established, Tarren Mill had fallen repeatedly and even changed hands several times. Although it was less important than Alterac Valley and Southshore, the worst thing that could happen to a defensive line was for the enemy to break through its center and cut it in two.
That would hamper communications and command, while also preventing the two ends from supporting each other. Thus, when one side's defensive line was pierced through the middle, it would usually retreat