There are several stories about the placement of Walt's name, including the one Granny mentioned.
The most logical one to me is that the names are meant to read like the credits of a film. The last credit usually goes to the director of the film, in this case, Walt Disney.
I also like the story that Walt's name is positioned to have a view of all the happy guests as they leave the Magic Kingdom.
It should be noted that Walt's is not the only name to face the castle. Across Main Street above Casey's Corner, several names face the castle, including Tony Baxter, Dave Durkhart, Ed Johnson, and Gary Younger.