I didn't say anything about being the most popular or used in the game and adding the D-23, a fine idea in a vacuum, wouldn't change that. There are Bf109s and Spitfires that are much bigger gaps because they have actual performance differences compared to the models in the game.
The problem you have with the D-23 is that it is just a D-25 that looks like a D-11. It doesn't add anything else.
If it weren't for the developer resources needed to do it that would be one thing, but there is such a tiny, tiny gain from the addition of the D-23 that it simply doesn't make sense to spend those resources there in my opinion.
Bf109s:
1940: Bf109E-4
1941: Bf109F-4
1942: Bf109G-2
1943: Bf109G-6
1944: Bf109G-6/AS, Bf109G-14, Bf109K-4
1945: -
Spitfires:
1940: Spitfire Mk Ia
1941: Spitfire Mk Vb
1942: Seafire Mk II, Spitfire Mk IX
1943: Seafire Mk III, Spitfire Mk VIII
1944: Spitfire Mk XIV, Spitfire Mk XVI
1945: -
P-47:
1940: -
1941: -
1942: -
1943: P-47D-5
1944: P-47D-23, P-47D-25, P-47D-40
1945: P-47M, P-47N