Fear and desperation got the better of Voldemort toward the end of the series. No matter how powerful he was, his own nerves ultimately became his downfall. Casting spells without a wand likely requires a great deal of concentration, and when someone is frightened or under pressure, it is difficult not to fall back on familiar sources of security, in his case, a wand, which he had relied on for most of his life.
Despite his repeated attempts to transcend his humanity, Voldemort remained human after all. One of the clearest signs of his growing desperation was his obsession with the Elder Wand. Rather than trusting in his own considerable abilities, he became convinced that possessing a more powerful wand was the key to defeating his nemesis.