Why was a CFM56 conversion never offered for the short-body DC-8s? A major part of why the DC-8 hung on well into the 21st century (albeit almost entirely in cargo service) was the late-1970s/1980s conversion of 110 DC-8-60s (out of a total of 262) into DC-8-70s; the main (or, for the DC-8-6/72 and DC-8-6/73, the only) change was the replacement of the DC-8-60's old, loud, thirsty Pratt &amp; Whitney JT3D low-bypass turbofans, and the nacelles thereof, with new, [relatively] quiet, efficient General Electric/SNECMA CFM56 high-bypass turbofans (and the nacelles thereof).1 2
Only the DC-8-60 was eligible, though; the short-bodied versions of the DC-8 (the DC-8-10/20/30/40/50), which formed a slight majority of all DC-8s ever produced (2943 out of a total of 556, as compared to the 262 DC-8-60s)5 needed not apply, even though they would have benefitted just as much from the reengine as the DC-8-60 did, and the process would have been mostly the same (in fact, the DC-8-61 was merely a straight fuselage stretch of the last short-bodied version, the DC-8-55, and, yet, the former version got [the opportunity for]6 a reengine, while the latter did not).
Why did the short-bodied versions of the DC-8 never get offered the CFM56 upgrade that was extended to the DC-8-60?

1: Thus continuing a long tradition of up-engining older-model DC-8s to newer-version standard, although the DC-8-70 was the only DC-8 version for which this was the only possible way of getting one (without any being built new).
2: The April 1979 decision to reengine DC-8-60s with the CFM56, incidentally, saved said engine from extinction, allowing it (in multiple later versions) to later see service on the 737-300-through-900, A320ceo, and A340-200/300.
3: Over half of these were of the latest and greatest short-bodied DC-8, the DC-8-50 (142 built new, plus a total of 20 converted to DC-8-50 standard from DC-8-10s [12 aircraft], -30s [5], and -40s [3]); there were also 52 DC-8-30s (not including the five converted into DC-8-50s), and smaller numbers of -20s and -40s (but not -10s, all of which [except for two which crashed first] were converted to DC-8-20 or DC-8-50 standard).4
4: I am aware that many of the short-bodied DC-8s had either crashed or been retired by the time the 1980s rolled around; however, this was also true of many of the almost-as-old DC-8-60s.
5: Numbers from here (Wikipedia, unusually for it, has had some major inaccuracies regarding the production and conversion numbers for the short-bodied versions of the DC-8, especially the DC-8-50).
6: Surprisingly, many DC-8-60 owners and operators did not take the opportunity to upgrade their aircraft.  Out of 262 DC-8-60s produced, only 110 were reengined into DC-8-70s; some of the remaining 152 DC-8-60s were presumably aircraft that had already crashed or been retired by the time the reengine programme came around, but, even of the DC-8-60s that were still around, many went unupgraded.
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.