There are a couple of reasons why the French did not attempt a massive attack to relieve the fortress.

First, the French High Command, specifically General Rene Cogny, who was in charge of the Northern Region, did not actually consider Dien Bien Phu to be a significant battle. Not only was it not in a strategic location, in involved only a small portion of the French Army in Indo-China (although it was the best portion). In fact, Cogny disliked the plan from the start and fought aggressively with his superior, Gen. Henri Navarre, over its implementation. Cogny considered the battle a waste as it drew troops away from the more strategically important Red River Delta region.Diverting enough troops to relieve the besieged fortress would have left Hanoi and the more important settlements extremely vulnerable, something Cogny would have found intolerable. The French were also conducting a separate military operation Atlante at the same time as Dien Bien Phu, further reducing the troops available to conduct such a breakout.

Second, you are underestimating the speed at which troops could move through the terrain. Mobilizing an army is an extremely complex task requiring exhaustive planning and preparation. Considering the area the French would be attempting to move through was extremely rugged undeveloped mountainous jungle their pace would be extremely slow. Although the Viet Minh were able to move about 30,000 troops and guns into positions (something the French had considered an impossible task). They did it over several weeks and months from November 1953 to March 1954. The French would have to move a larger, heavier force, a greater distance. Planning and execution would have to occur in a matter of weeks for it to be effective. Given the terrain of North-west Indo China this would basically be impossible.

This relief force would also come under concentrated attack by the Viet Minh who had proven very adept at ambushing French columns. Three years earlier, at the Battle of Colonial Route 4, a withdrawing French force had been nearly totally destroyed by Viet Minh attackers. Since the Viet Minh would know the goal of any relief force, it would have been easy for them to mine, ambush, and harass any French force attempting to move towards the fortress. The relief column did not need to be destroyed, merely delayed.

Finally, the French did attempt a relief force, Operation Condor, originally designed as a way to pursue the defeated Viet Minh, was hastially re-written in mid April as a relief effort to save the fortress. Although the French developed an elaborate plan in the end they were able to commit about 4 battalions of infantry and Lao partisans who would move east from Laos to attempt to reach the fortress in time to attempt a breakout. However, the plan relied on a large airborne contingent dropped around the outside of the fortress as you suggested. Before the ink was dry it was clear this was unworkable. For one thing any remaining paratroopers were already either in the fortress or about to be dropped into it leaving the air mobile portion significantly reduced. For another, Dien Bien Phu now consumed pretty much all available air cargo transport the French had, and it was stretched to the limit. The difficult terrain meant air supply was pretty much the only option and it would have to be scaled down even further. To be effective, Condor still relied on an Airborne group drop to reinforce the lightly armed partisans at the head of the column and by April 29th it was clear, due to supply, weather, and the deteriorated situation in Dien Bien Phu itself, that no paratroopers were forthcoming. Although lead elements of Condro did reach the fortress it was literally hours before it fell and they ended up retreating back into Laos.

Primary Source: B. Fall. 2002 Hell in a Very Small Place De Capo Press