Base layers can only be displayed in 3D View as Vector Data if they contain at least one XY point, and that point has a numeric attribute (to be used as the Z coordinate). Other objects within the base layer such as polygons, polylines, and points without numeric attributes can only be displayed in the 3D View as a texture on a surface. Point cloud maps and point cloud layers are not surfaces, they are instead a collection of points. Therefore, there is no surface for the base layer to be mapped onto. To display point data with numeric attributes as vector objects in 3D View, they can be in a base, base from data, post or classed post layer.
If you wanted to display a base layer as a texture on LiDAR data, you could grid the LAS or LAZ file, generate a contour or color relief map, then click Map Tools | View | 3D View. Details on how to do this are in the following knowledge base article: I create a profile through my point cloud, but I don't see anything. Why?
Updated February, 2020