Intel segments ECC support by Chipset and only the Workstation chipsets get ECC support. I recall reading somewhere that Intel was also ceasing the support of i3s and server chipset support and the Intel Ark page supports this: https://ark.intel.com/products/126688/Intel-Core-i3-8100-Pro...
Probably because there are no i3 equivalents in the Xeon line (no dual core Xeons) and Intel wants a low-end ECC supporting CPU.
Keep in mind that for at least the last couple generations, even though Celerons, Pentiums, and i3s support ECC you must use a server chipset like the C23x or C22x to actually get ECC on those CPUs.