"professionalism" can mean many things. In this context it can be broken down to two things: Money, Delivery

Some have a rare talent of delivering a song in such a way as to grab the attention of an audience. Some professionals can give a flawless performance on a recording, but live performances can suggest either they had an off day, or the sound desk massaged the recording extensively before release.
Drive, ambition, talent, luck all play a part in the transition to professional for making money.
To what extent these same factors create the gifted amateur, who knows?