Besides having larger (7- and 9-inch) CRTs to work with, well-designed, ATSC-based RPTVs are not encumbered by the inclusion of a shadow mask (or, in Sony's case, an "aperture grill"), which inherently limits resolution on direct-view sets.
On the other hand, because direct-view sets possess higher light output and a greater contrast ratio, they can appear to be sharper and more resolved. But that's an illusion.