So, I can't answer why they got removed, that's on Apple. But, here's how you can get them back. Not on your iPhone/iPad directly, you need to use the computer you have synced the phone with. Use Google (or whatever) and search for the stations. Click the link that it finds: for Top 50 Pop it is https://itunes.apple.com/us/station/itunes-weekly-top-50-pop/idra.691049516. There's a button to Listen in iTunes. Click that, it will prompt to confirm an External Protocol Request (basically you're opening a stream in an application outside of the webpage). Click on Launch Application and iTunes will start streaming it. Better still, this is now in your Recently Played radio station list and will be available to you going forward. You may need to sync the phone to get it available there too.
It's annoying, but at least they still exist and can be put back on the phone.