From a different perspective, it appears Zerotier is investing a lot of engineering efforts in maintaining its own protocol, with arguable benefits. Tailscale seems to be deploying new features on an almost weekly basis and are having huge velocity, and to me it just appeared their trade-offs made more sense, and I have more confidence in their business doing great on a long term.
Tailscale has captured mindshare, reminds me some of Cloudflare. There are many solutions out there offering unique value props relative to what Tailscale is doing, and it all comes down to customer needs. I'm not sure how broadly Tailscale has been adopted outside of the dev / HN crowd, so that's always something to dig into.
In all honesty, Tailscale being so good, we never seriously looked at Zerotier. We only evaluated it on paper, not actually tried to use it.
Being able to run Tailscale under otherwise very restricted userland environments, without changing too much code is nice.