Apple joined Google and Amazon in the home assistant market today. By leveraging Siri and HomeKit Apple will become a major player in this market. Amazon leads the market by virtue of being first to the game and Google will be able to leverage its massive data to be a competitor too.
For Apple and Google the presence of their assistants on smartphones makes it easier to persuade customers to stay within their ecosystems. If you're using Siri on your iPhone why wouldn't you choose to use Siri at home?
By abandoning Windows Mobile, Microsoft has put itself into a position where Cortana speakers are going to fail. Pushing Cortana on the desktop just doesn't help, because our PCs just aren't as personal as our phones.
There's just no easy sell for Microsoft's partners here. Amazon has the low end market sewn up, Apple is going to take over the premium market leaving Google to take the middle ground. Trying to sell a HP or Samsung speaker with Cortana - which no-one uses or cares about - is going to be virtually impossible.
For all Microsoft is focusing on the next big thing, it's hard to see the next big thing not being driven by your smartphone. VR, home assistants and home automation all have that link. That's going to be an incredibly difficult challenge for Microsoft to overcome.