Actually, Excalibur is a fairly traditional version of the Arthurian myth...strongly influenced by Mallory, I believe. I like the movie very much, except the Search for the Grail section gets a bit tedious.
I have theories as to why...with the exception of Camelot, the Musical...the Arthur myth has not been more successfully exploited by movies...It just does not seem to be a subject matter that the movies have gone to the well to all that many times.
I think partly it is because it is a "dying fall", which is why it is a great story to me...but tough to graft a Hollywood happy ending on for the studios.
But I think the biggest problem is that Arthur just always comes off looking like a complying cuckold...a very tough position for a protagonist to overcome. Also if you read the traditional legends, after his early adventures...sword in the stone and all that...Arthur's central position in the tales somewhat fades in the background once he becomes king. He comes on strong at the finish, but he has not second act...which spells doom for so many scripts in Hollywood.
There are authors whose alternative versions of Arthur, departing from Mallory and Tennyson, and embracing the legends' far more interestng Celtic and pagan origins, devoid of all that sanctimonious Chritstianity, have captured much more interesting portraits of Arthur --things like Mists of Avalon by Marion Zimmer Bradley or Firelord by Parke Godwin or The Great Captains and The Green Man by Henry Treece.
It's a subject I find fascinating...having about three bookshelves full of Arthurian related material.