I think think the simple answer to that one would be tooling costs. I heard a slide for the moulding of the nose clips on the western would cost thousands, so at that rate you would need a small fortune to make slides for all the 56 variations. Which would only push up the price we pay.
To me, the Farish 56 didn't look like a 56, however, in my opinion, the Dapol 56 looks like a 56 and although it may take some work to accurately model the variations for your particular loco, it is an excellent starting point.
Personally i can't wait to get my mitts on them!
Cheers
Mark