Well, there are two sides to this. The French underestimated their enemy and the Vietnamese were able to surprise the French.

The rational of the French was, that they wanted a Base they could support by air, hence in the valley and not on the hills, which they thought they could control due to their superior equipment. But most importantly, the French were under the impression, that the Vietnamese would not be able to bring heavy guns and mortars into the battle. They thought this because the Vietnamese relied on men and animals for transport since they had no access to the logistics of a modern army. Due to the terrain (thick rainforest and hilly landscape, and all that) the French thought it impossible for the Vietnamese to transport any heavy guns and mortars on the back of donkeys and by caring them on their backs. The French thought all the Vietnamese could muster would be smaller guns and mortars. The French base, after all, was well equipped to deal with light mortars and small guns fire.

But in come the Vietnamese. And they managed to bring heavy guns and big mortars in great enough numbers to inflict damage the French were not prepared for. They also managed to damage the runway greatly which cut the French supply line. Incoming supplies and soldiers had to be parachuted into the base, while transporting wounded soldiers out was only possible in the beginning of the battle. Not any more once the runway was damaged beyond a night’s worth of repair work.

To summarise: it was not really French hubris that lead to this disastrous decision but rather incomplete intelligence and gross neglect of the capabilities of the Vietnamese that led to this decision.

Despite me drawing from 7 years of under- and post-grad studies in which I touched on this topic several times I’m afraid I don’t have any literary sources on this topic at hand and thus would be grateful for anyone to add to/ diversify my answer.

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edit: orthography and clarification