Why were most of the first-generation-Learjet wing mods never approved for the Learjet 23? First-generation (straight-wing1) Learjets (the 23, 24, 25, 35, and 36) are, rather infamously, beasts to fly, due in part to their hideous stall behaviour;2 due to their flow-separation-prone laminar-flow wings, they have a tendency to stall at a fairly high speed with little to no warning and roll uncontrollably to one side, something that caused a considerable number of fatal crashes upon landing.

For this reason (and others), a number of modifications were devised over the years for the first-generation Learjet wing to improve its flight characteristics:


The Dee Howard-Raisbeck Mk. II wing mod, which lowered the aircraft's stall speed considerably, was released in 1975.
The following year, Gates Learjet responded with the Century III modification pack, which both lowered the aircraft's stall speed (by modifying the cross-section of the wing's leading edge) and modified the aircraft's stall-warning system to alarm earlier if the aircraft's angle of attack is increasing rapidly.
In 1979, the Softflite modpack (a set of stall strips and wing fences installed on the upper surfaces of the wings) became available; it considerably improved the straight-wing Learjets' stall characteristics, changing their behaviour from an abrupt stall and violent rolloff to a gradual, wings-level stall.


Learjet 23 owners, however, are unable to benefit from most of these improvements, as most of them were never approved for use on the 23.  The Mk. II modification was approved for the Learjet 23, but the Century III and Softflite modpacks - with the former's improved stall-warning system and the latter's benignified stall characteristics - were made available only for the Learjet 24 and up:


  The manufacturer's Century III and Softflite wing modifications to improve the airplane's slow speed and stall characteristics have not been approved for the [Learjet] Model 23.  The Dee Howard-Raisbeck, Mark II, a similar wing improvement modification, has been approved for the Model 23, but the accident airplane was not equipped with this modification.  ...  [NTSB AAR-83-01, page 16 (numbered as 13).]


Why was the Learjet 23 excluded from most of these stall-protective modifications?



1: Technically, the wings of first-generation Learjets do have some sweep.  However, the wings' sweep angle is too small to make a meaningful improvement in the aircraft's flight characteristics; the 23/4/5 have the leading edge swept at 13° and the trailing edge unswept (giving a quarter-chord sweep angle of only 9.75°), and the wings of the 35/6 are based on the 23/4/5 wing, with the addition of completely-unswept wingtip extension panels which give it an even lower effective sweep angle.

2: Other bad points: an original autopilot with a great tendency to produce large, spontaneous pitch inputs in either direction, a tendency to dutch-roll violently if said autopilot failed or was disconnected at cruise speed, or if the aircraft's yaw damper failed or was disconnected at any time except when configured for takeoff or landing, a mach-tuck susceptibility best described as "extreme" at not far above MMO on an aircraft with easily enough engine power to exceed that limit in level flight (aggravated by an overmach-recovery procedure which, as it turned out, was very likely to send the Learjet into an unrecoverable dive), et cetera, et cetera.
The cost of approving an STC for an aircraft can be quite expensive and the return on investment may not have been worth it.  

------------------------------------------
|Aircraft   |  # Built |  Year Certified |
------------------------------------------
|Learjet 23 |    104   |       1964      |
|Learjet 24 |    259   |   1966 to 1976  | (All variants)
|Learjet 25 |    369   |   1967 to 1976  | (all variants) 
|Learjet 35 |    670   |   1974 to 1976  | (all variants)
|Learjet 36 |     64   |   1974 to 1976  | (all variants)
------------------------------------------


The Lear 36 is, for all practical purposes, a variant of the Lear 35.  If you take a look at the list, the Lear 23 is the oldest with the least number built and that would weigh heavily on whether or not to pursue an STC.  If an owner of a Lear 23 wanted to purchase the STC and absorb the cost of the certification, these would have been made available to those aircraft as well.    

An owner can effectively upgrade their Lear 23 to a Lear 24 by completing and engineering change record (ECR 227, ECR 230 or ECR 233 depending on serial number).  Once the new Lear 23/24 name plate is installed, the STCs may be applicable.

It is also just a guess.  The FAA (as we know it now) was formed in 1967.  It is possible, the STC holders tried to get their modifications approved by the FAA for the Lear 23 but since the aircraft was certified before the FAA was formed, they may have declined or required the aircraft to meet current aircraft design standards and that may have been cost prohibitive.