Morna Hooker, in her commentary on Mark, points out the irony. Throughout this gospel, Jesus has been telling witnesses not to say anything about what they have just seen, and they disobey him. Now, when he says it is time to tell, the witnesses are too scared to say anything.
Some ancient authorities end the Gospel of Mark at this point. Others add verses telling that the women did, after all, tell the disciples. Still other authorities add other verses relating an appearance by Jesus to Mary Magdalene and that nobody believed that he was alive and that she had seen him. He also appeared to the eleven disciples and upbraided them for that unbelief.