Why was a CFM56 conversion never offered for the short-body DC-8s?
I suspect it's because the business case just wasn't there. Every retrofit project I've been involved in boiled down to the money. Retrofits aren't cheap and ultimately the mods have to recoup those costs through improved revenue or decreased operating expenses.  

Looking at your references, there was some structural changes to the wing and landing gear that came on the -55. I expect the earlier versions were ruled out of the mods due to the lack of those upgrades and the heavier CFM engines.  

That doesn't explain why the -55 wasn't included. The Wiki article stated the -61 conversions did cost more than the -62 and -63 conversions because it had the shorter wing of the -55. So it boils down to the difference between the -55 and -61. Unmodified, the -61 had about 38% more payload capacity (72k lbs vs. 52k lbs). But the modified -71 lost almost 12k lbs of payload in the conversion due to the extra engine weight. So the -71 advantage over the -55 was only 20%.  

The -72 and -73 didn't lose as much payload with the mods as they had the longer and stronger wing to start with. If you apply a weight penalty to the -55 similar to the -61 mods, it cuts the payload to approximately the same as the -43. That's about a 20% reduction in payload.  

The other consideration is who was doing the mods.  I didn't do an exhaustive analysis, but the vast majority of the -7x conversions were for the big air cargo operators - Flying Tigers, UPS, and FedEx - or large leasing companies where the planes were in use in the US and Europe where noise was becoming an issue.  

Many of the -6x aircraft that weren't converted (discounting those written off or broken up) were being operated in the countries where cheap trumps everything.

So most of the conversions were for cargo.  Another plus for the stretch aircraft is that cargo operations tend to max out on volume before weight. Obviously, that's not always the case, but the loads that max out weight tend to be special cargo (heavy equipment or machinery) not routine packages. The -71/-73 could carry almost 80% more volume than the short bodies.  Even the -72 had a 16% greater cargo volume. That's a big advantage in operational efficiency.  

Since the long body could haul more cargo and generate a greater margin per flight, it made a worthwhile investment.