Id think one of the big issues is the lack of anything easily comparable. With something in preservation, you can take all the photos, measurements, and paint samples as you want. For us lowly PreGrouping modellers, we have to make do with sometimes basic drawings, limited photos of varying timeframes, and lacking scratch-aids.
Id love to build a CR 670 class as I love the look of it, but I cant find anything better than a basic side view line drawing! Let alone making it to RTR quality levels. No wonder the major manufacturers wont try it.