MR doesn't quite have that same ring to it as VR.
VR is a tech that will stay regardless of future tech emergence.
But MR is an evolving next step beyond VR. Devices being designed such as Hololens & Magic Leap are a totally differen't tech from VR and they will want their products/content to be distinguished from VR.
Microsoft are branding around both "holo" & "mr" because they want to put their product in a category that they will promote as being a step further than virtual reality.
It will be content that is in and can interact within "your" reality.
It is the headset companies themselves and tech industries that are pushing the MR keyword, they have got the option to brand around VR, but instead have chosen the buzzword that describes their technology and are insistent that they want it to be referred to as specifically "MR".
I am not a "MR" pusher, I have mostly VR names, a fair few "holo" domains, a few "AR" & "MR".
A few members of this forum have really dug deep in their research, contacted relevant companies for even an insider perspective and it all points to "MR" being the keyword of choice. Microsoft & Magic Leap themselves have insisted that they don't wan't to be associated with AR, MR is the term that defines what they are aiming for.