Microsoft disables Cortana for Android voice feature after user complaints
While reviewers are frustrated that they can't activate Microsoft's virtual assistant using just their voice, it seems that the feature's removal was actually done to mitigate a bug. According to Microsoft, when "Hey Cortana" was enabled, people were unable to use Android's systemwide voice-search feature.
In other words, people who wanted to talk to their phones using both "Ok Google" and "Hey Cortana" weren't able to. Running Google Now and Cortana side-by-side may seem strange, but Google's voice-activated search can work inside any app. There's also the matter of Cortana using Bing as its default search engine -- people who prefer to use Google for finding stuff on the Web may want to have the option of using both assistants.
Interestingly, that issue wasn't found during the beta process for Cortana on Android. Users have taken to the app's Google Play reviews page to decry its removal, with some going so far as to call it "useless" without the ability to invoke Cortana using their voices.
It's not clear what the timeline is for the feature's reinstatement (or if there even is one) but given the incredibly negative user response to Microsoft's removal of the functionality, it would be surprising if it doesn't reappear. Furthermore, if Microsoft plans to make Cortana a reasonable competitor to Google Now, having that feature available would be a no-brainer.
At this point, though, it seems unlikely that the functionality will reappear before 2016, since Microsoft probably won't want to re-enable "Hey Cortana" before engineers go on vacation for the holidays.