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.
https://ark.intel.com/products/97455/Intel-Core-i3-7100-Proc...
The latest i5 or i7 with ECC were introduced in 2014.
Now, does anybody know why i3 but not i5 or i7?