Why wasn't Darwin (open core of macOS) more of a big deal?

Because there's not really much special about it, among a number of other open source and free OS kernels around (Mach, L4, Hurd, *BSDs...).

Linux is hugely successful because of the amount of available integrations and add-ons that are similarly free software.

Perhaps, novices needed Darwin operating in a bare metal environment to start tinkering. It would be nice to compare Darwin and Linux in an seL4 environment as guests and measure, explore practical, theoretical performance dynamics. I'm curious to know how Darwin's memory management compares to Dragonfly.