>>534548627
She failed because Ashenvale was already under siege, and it's the only way leading to Azshara.
You can't move troops over Mount Hyjal and the northern sea is generally frozen over, sailing all the way around would have taken months and leave your troops open to shore attacks halfway through.
The only way to secure Azshara would have been an armada coming from Gilneas building a bridhead and then flanking the troops sieging Ashenvale to allow the Nelfs to build a supply line, but they had their own problems around that time.