In a post on its support site, the company explained:
Unfortunately, Windows Phone has far fewer users compared to Android and iOS. This means that the market is relatively small. It is always a good idea to check out the future possibilities of a platform before purchasing a phone.
Like every other company, we make our decisions based on where we see the greatest potential. At the same time, we are one of the few companies that has developed apps for this platform. Naturally, we also want to offer our Windows Phone users the best possible products. However, we hope you understand that our priorities are set according to platform and market and that from now on we will no longer be updating our apps for Windows Phone.
The app is still available on the Windows Store, although it's not yet clear if Runtastic intends to remove it from the Store, or how long the app will continue to be supported in its current state on Windows phones that already have it installed. But with no further updates on the way, existing users will no doubt be seeking alternative apps... or alternative handsets running Android or iOS.
Declining interest in Windows phones among new buyers - and existing owners replacing their devices - has made it increasingly difficult for companies to justify their support for the platform. According to IDC, Windows' share of global smartphone shipments dropped to just 0.4% in 2016, and the latest sales data report from Kantar Worldpanel ComTech this week also drew attention to the "decline in Windows" phone sales.
When Amtrak ended its Windows phone support in August, it noted that "less than 1%" of its users owned a device running Microsoft's mobile operating system.