Cost, mostly; maybe pincount and routing at some level as well (I think current connectors have pins for ecc, but maybe this enables some pinsaving for ddr6 or the new ram connector from Dell?).

But also maybe performance? ECC ram usually doesn't come in high performance models, but with in-band, you can use whatever ram. Clearly, there is a performance penalty for in-band vs no ECC with the same ram, and there's almost certainly a penalty for in-band vs side-band at the same ram timings, but is there a benefit for in-band with faster ram vs side-band with conservative ram? Maybe? need approrpriate benchmarks.

If this has similar reporting to machine check exceptions that proper side-band ECC supports, this may be an easier path to get ECC on low end machines than hoping all the stars are aligned for ECC support and then paying a significant premium for the ram.

DDR5's on chip ECC probably increases reliability, but also means failures will be worse. You'll always have at least two bits flipped if bits are flipped on the chip, because single bit errors will be corrected. It's not clear what will happen with errors that are detectable but not correctable, since, AFAIK, there's no mechanism to report errors.