Well, the reason why big dinosaurs could had become less diverse is either because the fossil record becomes more scarse in the late Cretaceous (it seems that, as the Mesozoic went on, less fossil sites formed, or that there are still undiscovered Maastrichian fossil basins), or simply because they were big; many dinosaurs, like tyrannosaurs, were highly precocial, meaning that they occupied several niches across their lifetime, and thus decreasing their diversity.

T-rex, for example, was actually America's main predator when it lived, and not even dromeosaurs were big enough to compete with it (by the Maastrichian, all american dromeosaurs were usually beaver sized). And let's not even mention pterosaurs, which were precocial as well (a Quetzalcoatlus, for example, could had started its life as a Pterodactylus like form and ended in a massive monster by the adult age).

Its worth to note, though, that mammals were always a bit towards the dominant side since the Jurassic, when aquatic, borrowing, gliding and even some carnivorous forms (which might actually had lived alongside a BIGGER mammalian carnivore), and its only a matter of time we find quite large mammals, specially in the poles, where their competitors, lepidosaurs and terrestrial crocodillians, were less common