I can see why he changed it from the point of view of someone who is pandering to over all pretty dense audiences. The CGI creature might be confusing to some as to it's origins. It's obvious to us, who have poured over these things like forensic scientists, but to the causal viewer?
I can also see why he might be inclined to use an actor because of the performance. Then again, we haven't seen this CGI monster in action. We don't know how well it emotes and if it comes off as a living, breathing creature on screen.