Eh, I don't much have a problem with that one. The Iron Horde did have advanced technology, but they were just two years after Garrosh had arrived. And Garrosh was an idiot. He was so hungry for revenge on Azeroth that he guided the Iron Horde to attack Azeroth too soon. The Old Horde had united, dealt with the Draenei, Magnaron and other threats on their world fully, before invading Azeroth. This time the Dark Portal was opened prematurely, before everything on Draenor was settled.
Also we did have help from our homeworld. We weren't stranded for long. The Horde and Alliance both brought in reinforcements through Ashran. And we didn't do it alone, but with help from the Draenei and several Orc clans.