The question at hand being, why did no one step in on the war being waged on the Kingdom of Azeroth?
The dwarves didn't seem to notice what happened, or other kingdoms, Kul'Tiras in particular was a trading nation with a great navy. Were they not trading with Stormwind at all?
How was it word did not spread to raise concern before the Kingdom was defeated?
The story, I believe, goes that the Horde struck so suddenly and out-of-the-blue that Stormwind never was able to send messengers, or that they never felt like doing it. Naturally, King Llane underestimated the orcs, thinking that Lothar and his men will be able to hold them off. So why bother the other kingdoms, if you're so sure of a victory? And by the time its victory was finally not assured, it was too late to call for aid, so Lothar's caravan/transport navy went up to Lordaeron after Stormwind was sacked.
If forces were ever called into Stormwind, it would've been minor as likely I think the northern nations wouldn't have believed tales of a 'great green tide of death' from the south from mere couriers or something.
That's my two cents.
Hm. I understand that, since Azeroth was the most powerful nation of men during the time. They would've thought they could win, but the war lasted even longer than the second war.
Stormwind after some time though must of realized they were outmatched, since they were losing ground.
Its just concerning that no other nation seems to even prepare for the conflict until Azeroth limps to the north with its tail tucked, "Hey guys, so, there are these things called orcs... They have these big things called ogres... And have dark magic... And want to kill us all."
Heck, maybe the other kingdoms were like, "haha, Stormwind is getting beat by a bunch of green gnolls or something." Only to realize the threat of the horde when it comes knocking on their door.
The Kingdoms seemed pretty independent of each other during the time. Elves kept to themselves, humans being humans are looking out for themselves. Only the dwarfs could've really helped Stormwind, but they belong in their mountains.
It is a bit odd that no other human nations prepared in the 5-6 years that Azeroth was getting destroyed, but from my understanding, most of the other kingdoms spare Ironforge (Who had issues with Dark Iron still at that point) really didn't care about Azeroth. Remember, it wasn't until Lothar, beaten and bruised came up to Lordaeron and convinced the Kings that they needed to unite to defeat this threat. And the only reason they really did it without question was Lothar was the last of the Arathi bloodline.
Never the less, looking at the Alliance of Lordaeron with the Kingdoms and so forth, you must remember even when working together, many Kings didn't like each other. Disdain for Greymane, hatred for Perenolde, and so forth.
In truth, Lothar's bloodline probably was the only thing that forced all of the Kingdoms into the Alliance.
Because it was orcs vs humans. Orcs vs humans, dwarves, and elves sounds awkward
I believe it goes that everyone, including the orcs, under estimated one another, so the Kingdom of Azeroth (Stormwind) basically said to themselves "Pfff we got this! We don't need no stinkin reinforcements!" Naturally, both sides took heavy casualties. Think of warcraft 1 as ww1: A whole hell of a lot of casualties on either side, and not a whole hell of a lot of allies.
This is also just one of those bits of lore that we have to accept was dictated by the earlier games, before any real thought of the outside world or the overall story was given. Warcraft, the first game, was orcs vs. humans. That's it, that was the idea for the game and that is what was made. The idea of other races coming in to join these two fighting forces didn't really surface until Warcraft 2, since that was the pitch for that game: no longer is it orcs vs. humans, it's Horde vs. Alliance with all of the crazy new allies that each gets.
I haven't read the novels for the first two wars, so this may be explained in one of them. I would just presume that Stormwind was too arrogant and/or stubborn to really bother trying to call for help.