Why Mark's Abrupt Ending?

I finished W.D. Davies' The Gospel and the Land.  Davies includes as Appendix IV an article by Gunter Stemberger entitled "Galilee----Land of Salvation?"  Stemberger is arguing against certain viewpoints about Galilee in the Gospels----that there was a Galilean church, for example----and his conclusion is that Galilee is emphasized in the Gospels because, well, that's where Jesus conducted his ministry.

But I don't want to talk here about the debates about Galilee.  What I want to discuss here is the issue of why the Gospel of Mark ends so abruptly.  In Mark 16, a man in a white garment appears to women at Jesus' tomb, says that Jesus is risen, and instructs the women to tell the disciples and Peter that Jesus will show up at Galilee.  But v 8 then says that the women told no one because they were afraid.  (Many scholars regard vv 9-20 as a later addition.)  Other Gospels present the women telling the disciples, and they also show the disciples having experiences of the risen Jesus.  But this is not the case with the Gospel of Mark.  Why?

What I understand Stemberger to be saying is that Mark does not want for his readers to focus on Jesus' appearances to a few witnesses in the past.  Rather, Mark wants for his readers to anticipate the coming parousia, and to realize that the risen Jesus still appears.

A professor I had at my undergraduate institution had another explanation for the abrupt ending of Mark.  My professor thought that the parable in Mark 4:26-29 was relevant.  That parable says (in the KJV): "And he said, So is the kingdom of God, as if a man should cast seed into the ground; And should sleep, and rise night and day, and the seed should spring and grow up, he knoweth not how. For the earth bringeth forth fruit of herself; first the blade, then the ear, after that the full corn in the ear. But when the fruit is brought forth, immediately he putteth in the sickle, because the harvest is come."

In this parable, a seed grows into a plant, but the planter does not know how.  Similarly, my professor said, Mark is saying that the Kingdom of God grows, even though we don't know how this happens.  In the case of Mark 16, how could the Kingdom of God have grown, if the women told no one about the risen Jesus because they were afraid?  Didn't the women need to testify to the risen Jesus for the church to even get off the ground?  Who knows?  It's a mystery!  God works notwithstanding human failure.  That was my professor's explanation of the abrupt ending in Mark.