Focused Inbox is one of the features that is not available for Windows while it has been on the other platforms from a while.
Microsoft has said this wont be the case anymore and that all the platforms would get the same support and feature updates.
According to Neowin, the lack of features in the Windows platform is because its a default app in Windows while its an app to download in the other platforms. Therefore being a default app it requires different experience.
Some of the new features that would come are the “@mentions” and “Cloud Attachments”. These new features would be available in the Q1 of 2017 while Insiders may get it pretty soon.
Source : Neowin