it is easy to get access to lordaeron, but to actually surround it, you need to guard the entire silvepine lake (that also had a foothold in one of it’s islands)
in a prolonged siege it can get reinforcements and supplies from the lake, which is not an option for the forsaken since they wouldn’t have anywhere else to get reinforcements from
but before third war there was dalaran and alterac nearby with access to the lake (and stratholme being a costal city very close that could come from the north beach)
but yeah, not a very defensible place, big fields in front of it is an invitation for a large army marching in with giant siege weapons, specially since there is no way to intercept an invading force from the north