Often it's license incompatibilities. Even open geodata is often licensed in a way incompatible with OSM. E.g. a typical attribution requirement would require OSM data consumers to include the full list of data sources in the attribution (which is usually just mentioning the OSM contributors). So often there are individual arrangements with the respective bureaus to give the data to OSM under different conditions. For example, the cadastre data here in a southern German state is available under the condition that the attribution happens in the OSM changeset, which is much easier to fulfill (and is being done automatically in editors as soon as you have that imagery as a background layer).
But generally, someone must care enough to ask the data holders to make such an import possible. And I guess for individual trees the interest isn't as high as for roads or building footprints.
Comments
Often it's license incompatibilities. Even open geodata is often licensed in a way incompatible with OSM. E.g. a typical attribution requirement would require OSM data consumers to include the full list of data sources in the attribution (which is usually just mentioning the OSM contributors). So often there are individual arrangements with the respective bureaus to give the data to OSM under different conditions. For example, the cadastre data here in a southern German state is available under the condition that the attribution happens in the OSM changeset, which is much easier to fulfill (and is being done automatically in editors as soon as you have that imagery as a background layer).
But generally, someone must care enough to ask the data holders to make such an import possible. And I guess for individual trees the interest isn't as high as for roads or building footprints.