I think it may have been a combination of things, mostly expense and knowledge of a good inexpensive recipe for red. As an example are there many(or any?) transparent red handgathered ground pontils from Germany or any red transparent Leighton melted pontil types?
On handgathered Slags not a whole lot of difference in cutlines. And you did get me to read the book again (Thanks Michael Cohill for a great publication). and did see reference to red in regards to the Oxblood marbles
Of course I would never state with 100% assurance that some transparent red marbles did not exit the MFC building. It is certainly possible. (but not probable) IMO.