Tractors and pans are much cheaper to purchase or rent than overhung rubber-tired scrapers. On longer hauls with good dirt, the scrapers will run circles around the tractors and pans. On small jobs with short hauls, the tractor pans do OK and run cheaper than the others. Single engine scrapers will require a push cat, the tractor pans usually do not. If you are to move a million yards a month, big scrapers are the only way to go provided the haul distance and material are right.
The tractor pans seem better suited to being laser controlled, and are better to be adapted with other attachments like discs, bee-gees, drags, or roller-compactors. You can rent out a big International or John Deere tractor to farmers for agriculture operations when earthmoving is slow.
Myself, I'd rather run a double-barrel scraper than just about anything else, besides my recliner that is.