These days it has to do with how it is written and produced, successful songs are often written with help of multiple songwriters and producers with an aim for the song to find itself on Spotify and other streaming service playlists, where it receives the maximum exposure. Chart music is driven by playlists.

Mainstream music isn't very complex any more, with songwriters and producers writing very simple, but catchy songs that are made for people to sing along or dance to.

So from my understanding of the music industry, a lot of the mainstream artist music is pushed by the record companies to the radio and social media. Also, using things like Apple Music and Spotify to promote their artist causing their music to go viral. However, one of the things I love these days is that artist don’t need these big record labels anymore to put their music out. Social media and apps like SoundCloud and YouTube make it possible for artist music to go viral. Sure some of these people are extremely talented like Adele, but she also has a big machine behind her getting her airtime all over the world

Melody. Rhythm. Theme. Appeal to specific cultures. But that's not a foolproof formula. I've heard great songs IMHO that didn't succeed. And it's only rare ones that go to billion plus views on YouTube. I can't see what's common between Gangam Style (Psy) and Despacito (Luis Fonsi), yet both went into that zone. I personally like L'ete Indien (Dassin) a lot and that stands at around 35 million views, way below a billion. A very popular Hindi song (Afreen) is at 200 million plus views but still nowhere near a billion. What is the logic? Nothing very clear, but beyond the so called tipping point things acquire their own self propelling momentum.

I don't know hit songs very well. Usually I just listen to old songs which are not that popular but very classic. I think a hit song or hit TV series must be unique and novel because audiences get bored easily. When I watched Game of Throne I was always shocked by the dramatic plots and that made me want to see more (not the last season...). GOT gave us a totally new and exciting experiece, and that may be one of the reasons why it was such a hit. BTY, I do like Adele's songs!