There are so many unknowns to those of us on the outside looking in, some of which you touched upon, but using the Monogram '36 Ford as an example...
1) What condition are the molds in? (good in this case. This kit's been reissued a dozen times or so)
2) Are there inserts for the original, stock wheels and tires (styrene, IIRC), or have the molds been irreversibly changed?
3) How much would it cost to change the molds/tooling back to the original issue version?
4) How many more kits would Revell sell in the original version compared to the sales numbers from the most recent "Car Show" reissue?
5) Would such a "backdating" make financial sense over the short term, or even over the expected life of the tooling?
My feelings? In the majority of cases, speaking of Monogram/Revell kits, I doubt it makes financial sense (i.e. money invested to backdate tooling is far greater than potential profits from said of backdated kit) for Revell, Inc. to do any backdating and/or major mold/tooling work.
I also think condition of the molds and lost/missing inserts is a major issue, too. My gut tells me keeping the entire inventory of Revell, Aurora, and Monogram molds/tooling organized is a full-time job for someone, and what about maintaining it all? Does Revell, Inc. have a staff of workers whose job it is to maintain the tooling, prevent corrosion, and repair molds if necessary?
I have to wonder if every single piece of tooling is accounted for and identified, so they know exactly what they have, and what can and cannot be used...which brings me to another thought- what if all the tooling for these kits you long for are either too far gone to repair, or have already been scrapped?
The other thing is that these "6 'n 1" and "3 'n 1" kits aren't really all that different from what Revell is offering today. Yes, you may not be able to get all the parts in one, single kit, but with the options we have available these days (this forum , eBay, swap meets), it's not too difficult to procure the extra, optional parts you desire. Often times the "Drag" and "Custom" tires are the same, the engine differences are minimal, and in the case of the Monogram '36 Ford, they're "doubling" the number of build options by including the greenhouse as a separate piece. That's kinda cheesy in all honesty, as I think most people would prefer the coupe and to have the top/roof molded as one with the body, instead of needing to fill and blend the gap between the two. It's really a 3' n 1 kit, with an optional greenhouse.
I think we older modelers tend to look back with our rose colored glasses, and our focus narrows in on what we want as older adults, versus what the majority of the model buying public wants.
Unless we can get answers from someone on the "inside", we'll probably know why certain kits are reissued and others aren't...but we can sure discuss it.