>>96860114
Because it's a call back to how the Eldar were first presented in Rogue Trader (as well as some old FW kits), their modern look is a thing they've already pinned down with the killteam, and they make for an easy small ally force that can be run alongside CWE or DE.
So basically nostalgia and convenience. Exodites are likely in the pipe, there's just more work to do before they're ready.