|Utahraptor on top, Achillobator below. Used with permission as noted on the image.|
Taken from A Field Guide to Mesozoic Birds and Other Winged Dinosaurs
Achillobator was an Asian animal, in modern context, and had a different landscape for a home. The Mongolia of 90 Mya was a semiarid desert, kind of like some of it is today. The Utah of Utahraptor, which is considerably older at 125 Mya, was a marshy mud flat with open tracts of land. The squatness of both animals was most likely an adaptation that favored the reduction in length of hindlimb to produce the ability to hide closer to the ground in the low scrub environments in which they lived. Utahraptor lived in a time devoid of large predators; allosaurids, ceratosaurids, and megalosaurids had all disappeared from the landscape during the existence of Utahraptor. Large carnosaur reintroduction into the landscape likely led to a loss of niche and subsequent dwindling of population and eventual extinction of Utahraptor. Achillobator lived alongside many other carnivores, though. Until we have a better range of existence data for Achillobator we cannot really surmise the events leading to its disappearance from the fossil record.
Reference of the day:
Martyniuk, M. P. (2012). A Field Guide to Mesozoic Birds and Other Winged Dinosaurs. Pan Aves.