nspawn (and LXC/LXD) is overlooked because people mostly want application containers not system containers.
1. Containers are commodity. For commodities, price wins first, then marketing. Docker is equally free (as in beer) and is far better marketed 2. There are a very low number of people working on systemd-nspawn and a very high number of people working on docker (and the ecosystem). 3. Dockerfiles and images are ubiquitous. They're easy to support from other run times, but if you're already in the ecosystem, what's the incentive to change?