Steam looks incredibly difficult to properly model compared to diesels. All those extra visible moving parts and the complexities of the cab alone make it confusing. Plus I bet it's more difficult to obtain reference material for a lot of those steam locomotives that aren't preserved and even some that are. Looking at the Big Boy render you can see how detailed it is already, but reference back to an image and just see how much further it has to go.
Hit the nail on the head! Just a litttttttle more complex than the diesels. If i remember correctly theres one sitting in Steamtown so it wouldnt take too much for me to make a reference trip.