Because it was just thought to be impossible, the war had already began at this point after the siege of Saguntum had extended more than expected and the romans were expecting an incoming invasion at any point. One thought was that an army would go through Massalia, invade the city state and arrive from the north, another fear was that Syracuse would rise against Rome and helped an invasion from the south while even another idea was that king Philip V of Macedonia would attack from the east. The threats could come from anywhere but at least they would know where they came from so the romans could send their armies accordingly before anyone could reach the City of Rome or its closest allies.

Hannibal crossing the Alps with an army cought the romans with theit pants off because they thought that something like that was impossible and didn't prepare for that which would lead for some of the darkest years for the republic of rome that had ever faced.

Why was it impossible? Well aside from being extremely cold, many passages where block because of the environment, there was hardly any type of local support for anybody that crossed by not even mentioning a whole army or that the logistical effort to maintain everyone alive and ready for years of war was a nightmare to get done... there is even something that could have ended Hannibal's expedition before it even started. The cohesion in his army.

Saying that Hannibal's army was multicultural would be a short sighted thing to say, there were warriors from almost everywhere; numidiams, iberians, gals.. a lot of different people from different backgrounds that didn't have anything in common or even spoke the same tongue. All of them under the command of the same man. Can you imagine how hard would have been to motivate all these people to go to war through the most dangerous route with little to almost no chances of survival? It was a feat of strength not only from Hannibal but from anyone that went with him, it was such the thing that had been accomplish that it is hard to belive that it even happened to this day.

And yet, the most incredible thing is not that enough of them made it through the Alps but the fact that they were willing to fight the romans in their own territory with, again, almost no chance of success.