This nature trust purchased the land.
Nobody is preventing a developer from buying it and building.
I can’t speak for CA but in these parts (CT) if a nature trust buys land it is stone dead for future development.
No developer would be allowed to buy it in this state.
The trust likely is not paying market rates on property taxes. They can hold it and never sell.
these nature trusts change the deed to ensure nothing will ever be built on the land, it is the whole reason they buy it to begin with.