Why does the term "sell out" have a negative connotation (if the whole point of making music is to make money)?

There is a huge difference between making money and only making money. The term sell out means that the artist is not making good music but is just concerned with making money. Often, some artists try to copy other celebrity styles rather than having their own particular style which also earns them the title of being a sell out.

This is because they lack originality and is simply trying to make money copying a more successful artist. A good artist sees their music as a career. Something they need to build and work on rather than a money earning machine.