Surely the police look at a case before deciding if it needs publicity or not. In the case of Joanna Yeats, they must have been able to tell early on that she had been abducted and was in grave danger, so they publicised it. And a few days later, her dead body turned up.
I don't think it's fair to say that she only got publicicty because she was white and well educated. She got publicity because the police knew somehow that she was going to be, or had been murdered.
If the figure of 750 people a day going missing is correct, it's safe to assume that the majority of them aren't being abducted and murdered. Otherwise we would all come across a dead body at some point in our lives.
In Serena Beakhursts case, the police probably have good reason to believe that she has not been abducted and murdered, hence it hasn't had much publicity.
It's that simple really.