A rift in the NTP world

AFAIU, the original ntpd implementation is important because of it's strong support for reference clocks. chrony depends on gpsd, and in general all the other NTP implementations likewise lack broad support for all the various reference clock hardware and drivers. This is why ntpd is still used so heavily as stratum 1 servers.

And I'd speculate that among the world-wide pool of stratum 1 servers, especially the reliable ones offered by public institutions, you'll probably find a ton of ancient, esoteric hardware and software (clock sources, operating systems, etc), which effectively binds them to ntpd and binds ntpd to them.