Because of the stigma attached to performing in what is seen as a somewhat tacky contest. Performers from other European countries consider it an honour to be asked to take part, and so their best performers are put forward for it. Over here, a lot of artists would think it a bit beneath them, maybe not in the early years with the likes of Cliff Richard and Sandie Shaw representing us, but more so nowadays. It can be hard to taken seriously as an artist, if you are known as a Eurovison performer.