Why were most of the first-generation-Learjet wing mods never approved for the Learjet 23?
The cost of approving an STC for an aircraft can be quite expensive and the return on investment may not have been worth it.  

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|Aircraft   |  # Built |  Year Certified |
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|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)
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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.