Why are some of the properties null in the result from getData?

That's because those properties are optional properties in the objects returned by the getData method. Optional properties are returned as null unless you explicitly specify in the request that you would like the properties to be included. You do this using the includeProperties parameter to getData. For example, to get the backlog items in each project, and the tasks in each backlog item, set includeProperties like this:

includeProperties=Project.backlogs,BacklogItem.tasks