From the ISC web site

There's quite a lot of interesting background/historical info on the ISC site, touching on both Kea and ISC DHCP, and their history, for example the blog article announcing the EoL for ISC DHCP (https://www.isc.org/blogs/isc-dhcp-eol/) - which points to a lot of the reasons why Kea was developed, and notes the following from one of the ISC DHCP maintainers:

"The relay was never really popular - people mostly used their ethernet switches features for relaying - but it provided value by being a “tool in the toolbox” so to speak, useful in a pinch."

And the relay and client functionality was EoL'd from ISC DHCP about a year and a half before the rest of the project, so it's perhaps understandable that ISC saw little demand for that functionality in Kea from their sponsors and other funding sources.

https://www.isc.org/dhcphistory/ was interesting too, particularly Kea's origins in BIND 10.

(Given the links to BIND 10, and that era at ISC, I'd suspect that there are probably some fun bits of history that didn't make it past the PR machine ;) )