Because cortana is not available in many countries. Plus, I do not want to speak to my phone when I'm in the train or walking with my friends. Every other OS has the functionality. Its been available since 2009.
Hopefully Cortana can call contacts from Viber!
There are situation / environment where cortana/ Siri can not be used practically. Personally I don't wanna talk to my phone while being in a public place.... More over, If other OS/competitor does have it why cant wp can not have it.
? aren't the idea of these PAs on WP,IOS & ANDROID to show the general public how they work so that people would be tempted to BUY the device that the PA they heard was found on? Just saying!!! If everyone used their PA in private, WTF are they still showing google v siri v cortana ads if NO-ONE actually experiences them being used in PUBLIC!!!