Why do genres change?

Some artists often change genres or try to blend two genres together for many reasons, one reason for this could be to follow a trend, for example the artist 'Chip' used to be involved with hip-hop and pop songs, whereas now that 'Grime' music has become more popular over the last few years he has been involved in more 'Grime' music to gain  popularity and to follow a trend. Another reason artists the genre of the music they produce is to change their image and to attract a larger audience. An example of this is Justin Bieber, his first music was a form of 'Cheesy Pop' with songs like 'baby' and now he's got older he's now trying to move into the 'RnB' genre.