First, singing requires a different use of the vocal cords than speaking. When we sing, we use our vocal cords to produce a sustained pitch, which can be difficult to do with an accent. Additionally, singing often involves using a wider range of pitches and vocal techniques than speaking, which can further obscure an accent.
Second, singers often take lessons or receive training to improve their vocal technique. This training may help them to develop a more neutral accent that is more suitable for singing. Additionally, singers may consciously try to lose their accents when singing in order to make their voices sound more pleasing or to better fit a particular musical style.
Third, the acoustics of a singing environment can also affect how a singer's accent sounds. For example, a singer who is performing in a large concert hall may find that their accent is less noticeable than if they were performing in a small room or an outdoor venue.
Overall, there are a number of factors that can contribute to a singer losing their accent when singing. These factors include the physical demands of singing, vocal training, and the acoustics of the singing environment.