#Because The Elder Wand was affined with Harry, not Voldemort
A wand chooses the Wizard. In order to choose one, it has to be given by the maker (like Harry was given his wand by Ollivander in PS) or be taken by force, won in battle (like Harry was able to use Draco's one in DH). In the whole series, we see several cases of borrowed wands that do not serve their users well.
Voldemort did not win The Elder Wand from its rightful owner (Dumbledore) - he got it from his grave. Draco Malfoy was supposed to be its rightful owner, as he had disarmed Dumbledore holding it (but did not claim it). But Draco was defeated by Harry, so the wand chose Harry and was reluctant to serve Voldemort.