I'm guessing it comes down to margins. So long as yields are good, making an 8 core CPU doesn't really cost them any more than a 6 core CPU, but they can charge more for it. At that point it doesn't make sense to release the 6 core part so long as you're still selling all the 8 core parts you can make reasonably quickly.
Edit: Although I guess the same could be said about non-X3D 8 vs 6 core chips as well. I'm not sure why it made sense to release 6 and 8 core simultaneously for non-X3D, but not for X3D. I still suspect they did so as a result of business considerations though, rather than not understanding that 6 core X3D CPUs would be viable products.
Well, I can see why they didn't do it with the R5-5600X3D because it was still unfamiliar territory but, from a business perspective, not having an R5-7600X3D was beyond stupid.
Remember that AMD was having difficulty attracting users to the AM5 platform because of the motherboard costs. Having an R5-7600X3D would've
definitely increased the AM5 adoption rate. From a business perspective, maximising the adoption rate of AM5 should've been their top priority based on what we saw with AM4.
The reason that AM4 was the best x86 platform ever released was its longevity. Once a user had adopted the AM4 platform, it made absolutely
zero financial sense to buy an Intel CPU for the platform's very long duration. For users, these drop-in upgrades were a major boon. There was no way that Intel could compete with AMD once a user had AM4 because even if the user had the budget for a new motherboard, an Intel CPU plus a $120 motherboard would get creamed by an AMD CPU that cost $120 more than the Intel CPU being considered. This is how AM4 brought AMD back from the brink and did severe damage to Intel at the same time. An AM4 user was a captive user and happy to be so. At the same time, the high adoption rate of AM4 really stuck the knife into Intel and gave it a few twists because Intel was completely locked-out of the huge AM4 user base.
The same business philosophy seems to have been applied to AM5 which means the more users that AMD could attract to AM5, the better off they'd be in the mid to long-term. After all, once a user has committed to AM5, it can be expected that their next two CPU upgrades will be AMD. That's essentially guaranteed revenue (never a bad thing) and the popularity of an R5-7600X3D would do even more damage to Intel, punishing them for making CPUs that rival/exceed GPUs when it comes to power consumption.
It's such an obvious thing but AMD managed to mess it up. The same company that showed amazing competence when producing AM4, RDNA and RDNA2 somehow managed the
complete opposite when producing AM5 and RDNA3.
I swear, AMD and ATi have become like Jekyll and Hyde.