One source of this problem is that only Google and Nuance have voice recognition engines that are any good, and they are very closed up (unless you pay tons of money to Nuance like Apple does for Siri).
Most developer's only option for voice technology is to use Nuance's API which requires uploading the voice sample and waiting for a response, this is nowhere near fast enough for pleasant interaction. Things will only get better when Apple, Google and Microsoft open up really high quality on-device speech APIs for their operating systems.