That is partly due to artificial reasons and partly due to technical reasons. The artificial reasons would be that the 8200 MT/sec UDIMMs are overclocked. Notice how they run much slower if you do not enable XMP/EXPO, which simultaneously over volts and overclocks them. These exist because a large number of people liked overclocking their memory modules to get better performance. This was unreliable and memory manufacturers noticed that there was a market for a premium product where the overclocking results were guaranteed. Early pre-overclocked modules required people to manually enter the manufacturer provided voltage, frequency and timings into the BIOS, but XMP and later EXPO were made to simplify this process. This idea only took off for non-ECC modules, since the market for ECC UDIMMs wants reliability above all else, so there never was quite the same market opportunity to sell ECC DIMMs that were guaranteed to overclock to a certain level outside of the memory IC maker’s specifications.

There is no technical reason why ECC UDIMMs cannot be overclocked to the same extent and ECC actually makes them better for overclocking since they can detect when overclocking is starting to cause problems. You might notice that the non-ECC UDIMMs have pads and traces for an additional IC that is present on ECC UDIMMs. This should be because the ECC DIMMs and non-ECC DIMMs are made out of the same things. They use the same PCBs and the same chips. The main differences would be whether the extra chips to store ECC are on the module, what the SPD says it is and what the sticker says. There might also be some minor differences in what resistors are populated. Getting back to the topic of overclocking, if you are willing to go back to the days before the premium pre-overclocked kits existed, you will likely find a number of ECC UDIMMs can and will overclock with similar parameters. There is just no guarantee of that.

As for RDIMMs having higher transfer rates, let us consider the differences between a UDIMM, a CUDIMM and a RDIMM. The UDIMM connects directly to the CPU memory controller for the clock, address, control and data signals, while the RDIMM has a register chip that buffers the clock, address and control signals, although the data signals still connect to the memory controller directly. This improves signal integrity and lets more memory ICs be attached to the memory controller. A recent development is the CUDIMM, which is a hybrid of the two. In the CUDIMM, the clock signal is buffered by a Client Clock Driver, which does exactly what the register chip does to the clock signal in RDIMMs. CUDIMM are able to reach higher transfer rates than UDIMMs without overclocking because of the Client Clock Driver, and since RDIMMs also do what CUDIMMs do, they similarly can reach higher transfer rates.

Thanks for the explanation on CUDIMM, I never quite grokked the difference besides it being more stable with two sticks per channel. Hopefully they'll make an ECC CUDIMM at some point, but I'm not holding my breath.

If they don't and you are up for a challenge in bga soldering you can make them yourself if there is pad for the chips. You likely have to buy an extra module to get the chips though.

This would also need a SPD programmer and possibly some additional SMT resistors, but it is possible in theory.