Why can't I download music on my iPhone 6?

So every single app I used on my iPhone 5 got downloading music does not work on my 6. It doesn't even let me download on apps like iDownloader, MyMP3, TDownloader, etc. Is there a reason for this? Are there any apps that I can download music from YT on? I go to the downloading app and copy the YT link into the youtube to mp3 sites and it won't let me download it. Can anyone help me out please?

Yes there's a reason. What you're doing is called stealing. This is the digital equivalent of going to store and stealing a CD. Now, unlike stealing a physical disc, no one seems to care about downloading an mp3 file without paying.

Apple got rid of these apps because they help you break the law. Apple would most likely be sued millions of dollars for something like this, so they got rid of those apps.

You can, however, use your computer and download your music, and put it on your iPhone through iTunes. It's inconvenient, but that's the price you pay for using Apple's "walled garden". If you had an Android device, then this would be much, much easier.