If you read on you will see that eventually they did tell, but at first they were scared and then once they had confirmation by having Jesus appear to Mary Magdalene, that they were bold enough to tell. Adding Matthews account to the fold you will see (Matthew 28:1-10) that it is only after meeting Jesus that they told the disciples. So the events of the day are as follows.
First they find an empty tomb then they are told that Jesus has risen by two angels, which cause great fear, then Mary Magdalene is met by Jesus and it is only after then that the women go and tell the disciples that Jesus is risen.