User blog comment:Morganaforever/Reviving your children/@comment-5674726-20121120192514/@comment-5674726-20121120214754

''However, I think the reason is that the producers probably hadn't planned to make Morgana Uther's daughter at that time and much less having Uther using magic to save her. It probably was done for plot-convenience.''

I wouldn't be surprised if there was also an element of making Uther look better by stressing his love for Morgana, and to make her look like more of a villain by comparison. I'd say that it's the same reason why they showed Morgana killing the helpful sentry in cold blood in The Tears of Uther Pendragon, given that the same episode revealed that Uther had little children drowned.