I don't think they are less popular.
But their whole system works different. There is not a single file there that's called Inception.h265.HDR.mkv for example
Its all just billions of g24hg54j2k7j6nb2n1n5b5j files with absolute gibberish as content. So you need the nzb files to actually get stuff out of it.
But the nzb files also don't hold any copyright infringing material in and of itself.
So copyright holders have to fight two thing at once
No, an invite is just so you can register. You still have to pay for most indexers. Or atleast you have to pay if you want to use their API access; And that's what you'd do if you use *Arr