It's because the app is not compatible with your device, and thus Google Play Store app filters those out because the user won't be able to install from it anyway (APK sideloading is always possible though).
As stated in Android Developer's official documentation,
How filters work on Google Play
Google Play uses the filter restrictions described below to determine whether to show your application to a user who is browsing or searching for applications from the Google Play app.
When determining whether to display your app, Google Play checks the device's hardware and software requirement, as well as its carrier, location, and other characteristics. It then compares those against the restrictions and dependencies expressed by the application's manifest file and publishing details.
If the application is compatible with the device according to the filter rules, Google Play displays the application to the user. Otherwise, Google Play hides your application from search results and category browsing, even if a user specifically requests the app by clicking a deep link that points directly to the app's ID within Google Play.
Note that app developer can also exclude individual device model manually.
Exclude your app from certain devices
Manage excluded devices per app
To prevent app crashes, you can exclude your app from being available to individual devices on Google Play. Making this change manually excludes your whole app—you can't exclude individual APKs.
By manually excluding devices with known compatibility issues, you can help provide a better experience for your users.
For this specific case, NextRadio Free Live FM Radio depends on the FM receiver chip on the user's device. The update since January 2, 2019 also states that it doesn't support streaming anymore and only use FM-radio.
NextRadio is going back to basics. Streaming is no longer supported, but you can still tune to your favorite local stations on FM-enabled devices.
On the other hand, Samsung Galaxy S4 doesn't have FM radio, and it will be pointless to install the app since it won't work properly. Note that the user can always sideload the APK if they want.