– because:
Because the official store is shut down, you must use developer tools to install these archives:
Download the Windows Phone SDK 8.1 Lite deployer or use WPV XAP Deployer on your PC.
Modern preservationists have created custom storefront alternatives that connect directly to community-hosted XAP and APPX repositories, mimicking the original marketplace experience on the device itself. How to Sideload XAP Files onto a Windows Phone
Finding the archive is only half the battle; getting the apps onto your legacy device requires a few specific steps. Because official developer registration servers are offline, you must use community workarounds to unlock your phone. Step 1: Interop Unlock the Device
If you want a (free apps only):