The major reason for the French loss at Dien Bien Phu was that they picked a really bad place to put their base. They underestimated what the Vietminh would be able to do to threaten the base.
The French placed a large base in the middle of a valley that was surrounded by hills. They thought that the Vietnamese would never be able to put artillery and anti-aircraft and other heavy weapons in those hills. Therefore, they felt, the base would be a secure place from which to prevent the Vietminh from taking over the Mekong valley.
However, the Vietminh managed to haul heavy artillery up into the hills overlooking the base. Once they had accomplished this, they could shell the French at will. They even prevented the French from dropping supplies in by parachute. Eventually, this siege caused the French position to be completely impossible and they surrendered.
They picked a really bad place to put their base, That is all there is to it :D