""reports say it interferes with Google Now's Ok Google functionality""
Read: Google "said" that only Google Now can use this function and all other software with similar function in PlayStore will be removed due to google license.
And because Microsoft is more and more popular software dev on PlayStore, it would rather remove this function that wait for google to remove whole app
I think one of the reason that MS removed the "hey Cortana" is because it can be easily mistakenly target while talking on the phone. I actually experience this while talking on the phone with my relative( and we are not even talking in English, and then i heard a woman answering me then i look at the phone screen and there is Cortana being open)