Isso é respondido no livro:
"We might go from there up along the River Running," went on Thorin taking no notice, "and so to the ruins of Dale-the old town in the valley there, under the shadow of the Mountain. But we none of us liked the idea of the Front Gate. The river runs right out of it through the great cliff at the South of the Mountain, and out of it comes the dragon too-far too often, unless he has changed."
"That would be no good," said the wizard, "not without a mighty Warrior, even a Hero. I tried to find one; but warriors are busy fighting one another in distant lands, and in this neighbourhood heroes are scarce, or simply not to be found. Swords in these parts are mostly blunt, and axes are used for trees, and shields as cradles or dish-covers; and dragons are comfortably far-off (and therefore legendary). That is why I settled on burglary-especially when I remembered the existence of a Side-door. And here is our little Bilbo Baggins, the burglar, the chosen and selected burglar. So now let's get on and make some plans."
(minha ênfase)
Em outras palavras, o Front Gate foi usado como um meio de saída e entrada para Smaug, e ele ainda era conhecido por sair frequentemente. Então, tentar entrar pelo Portão da Frente corre o risco de encontrar um dragão vivo, que era um risco que nenhum dos Anões estava preparado para tomar (particularmente dada a ausência de Guerreiros e Heróis).Sabendo que existia uma entrada lateral, eles decidiram seguir o caminho mais furtivo.