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.
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.