Writing a hit

Well, there really isn't a rule about what makes a hit song. When it comes to hits that derive from musicals, there's really two types of integrated musical: 1. An integrated piece that has what Rodgers called mechanicals, but also has a few songs that work in the context, but can easily be taken out of the context and done effectively by a jazz or cabaret or (god forbid) rock singer. They Say It's Wonderful is a really good example of that. Who would know that it comes from a Western musical about Annie Oakley?

2. The Sondheim revolution. An integrated show where the songs are so tightly devised as an extension of the scene that it's almost impossible to really do them unless the audience is familiar with the show.

Otherwise, it's just about happening to have the right sound at the right moment, and a record company or fanbase that's ready to publicise the song.