DDR5 "on die" ECC is not the same a proper memory level ECC that we are used to. Sure it does some stuff, but it is not a 1:1 comparison. It does not protect your actual data:
As to why, could be it was getting expensive, or AMD changed something in their chipsets due to DDR5 being partial ECC?
Also with some AM4, while it supported ECC, it did not mean that it actually detects orcorrects any errors, it just is able to run it.
One user did not some tests were done with AM5 and ECC worked..
This person notes it did work (there is a command you can ruin to verify if ECC is actually enabled)
And if you want to get into a dam long thread of who's d**k is bigger about DDR5, speeds and such