Before we get into potential causes of the delay, I think it's important to say that the key focus of replacing CZ-2,3,4 wasn't to prevent booster on village syndrome it was to;
- To design a series of launch vehicles rather than one launcher made for a specific mission, so that to enhance China’s capability of entering the space.
- To apply the advanced technologies, such as the large diameter core and the powerful thrust engine to increase the launch capacity dramatically, with the goal of launching 25-ton payloads to LEO and 14-ton to GTO.
- To design a series of launch vehicles based on the principle of generalization, serialization and modularization, with the purpose of meeting the needs of launching different payloads.
To use non-toxic and non-polluting propellant.
- To be low cost, high reliability, and convenient for test and operation.
The most obvious cause of delays is the CZ-5 failure on its second flight in 2017. It was only able to return to flight 2 years later in December 2019. However the cause of that failure, YF-77 turbopumps isn't a problem for the rest of the newer generation launchers as they don't use it. And the CZ-5 is new capabilities; it's not replacing anything. So no simple explanation for me.
I think the next thing to address is what is replacing what. The specific launch vehicles that are still launching and need to be replaced are CZ-2C, CZ-2D, CZ-2F, CZ-3B and CZ-4B This (translated) forum post gives a pretty alright summary of it. At a guess; this is the rough list of replacements;
CZ-2C --> CZ-6A / CZ-8
CZ-2D --> CZ-6A / CZ-8
CZ-2F --> 921 single core
CZ-3A --> CZ-8
CZ-3B --> CZ-7A
CZ-3C --> CZ-7 (maybe 7A)
CZ-4B --> CZ-6A / CZ-8
CZ-4C --> CZ-6A / CZ-8
A key issue is that the CZ-6 is too small for the CZ-2 and CZ-4 payloads, meaning that the larger CZ-6A needs to be developed, which boosts capability with 4 SRBs and a larger second stage (I think the second stage on the regular CZ-6 is a bit smaller to avoid boosters falling on populated areas). The CZ-6 is more of an alternative to the CZ-11 and is thus a small sat launcher than a replacement for MLV CZ. The CZ-8 was also designed to launch these payloads; however in 2018 it was redesigned to be support the derivation of a reusable launcher. This meant the YF-100 had to undergo dev to allow for deeper throttling as well as the change from solid to liquid boosters. (note that the upcoming launch in December will be the CZ-8 not the reusable CZ-8R)
CZ-8 and CZ-6A will probably fill similar niches, which comes as a result of being made by different SOEs, SAST making the CZ-6 and CALT making the CZ-8. (bit of competition here, especially for who gets first China VTVL)
Well what about the CZ-7? A couple things; they looked to improve reliability after the first 2 launches, they had to develop vertical intergration and I think to replace the CZ-3 series, it needs a restartable upper stage for GTO launches; which meant they had to stack the hydrolox CZ-3 upper stage on top, forming the CZ-7A. This last reason is likely the main driver in delays. The recent failure in March, which I believed was caused by dodgy wiring causing the YF-100 on the first stage to reignite after MECO impacting the second stage which will result in the delaying of replacement.
921 and the next gen crew capsule are still heavily in development, so CZ-2F in the short term will still take astronauts to the upcoming Chinese modular space station.
So in summary, the CZ-2 and CZ-4 series have yet to be replaced because CZ-6A and CZ-8 haven't yet finished development. The CZ-3 series is yet to be replaced because the CZ-7 still had to undergo development after its initial launches to be able to replace the CZ-3. CZ-2F hasn't been replaced yet because 921 is still under development (2025 debut is the number I see chucked around for it)
Basically, the required rockets haven't finished dev yet.