Ecological speculations: They are large enough that they likely have few predators, since they would probably be able to fend off most carnivores with a powerful kick or a lash of the tail. It is possible that the brachiosaurs that were shown to live in herds in "Genesis" were females, while the lone one seen in "What Remains" is male, a social difference seen in modern day elephants.
The dinosaurs were only referred to as "brachiosauri" in "Genesis", so they were possibly not intended to be the specific genus Brachiosaurus, but just to be a member of the broader Brachiosauridae family. The exact genus or species of this Brachiosaur is currently unknown, but it may be from a known genus of brachiosaur that lived in the Cretaceous, such as Abydosaurus, Paluxysaurus, Pelorosaurus, Sauroposeidon, or Sonorasaurus. This is a more likely genus than Brachiosaurus, which lived during the Jurassic Period. Sonorasaurus in particular is known to have lived in a section of the Cretaceous from 112 to 93 million years ago, closer to the time of Terra Nova (85 million years ago), while the other genera are only known from fossils in the lower Cretaceous, from around 146 to 100 million years ago. All of these genera were from North America or Europe which had broken apart from South America (where all Carnotaurus fossils have been found) in the Middle Jurassic, but there are also some fragmentary remains of a Brachiosaur from Africa (which was joined to South America until the Early Cretaceous) dating to the late section of the Early Cretaceous, about 112-100 million years ago. These remains were originally considered to be a species of Brachiosaurus, and were assigned the name Brachiosaurus nougaredi, but this was when the fossils were thought to be from the Jurassic and later paleontologists have considered it to be a different (unnamed) genus. The dinosaur on the show might simply be intended to be an unknown genus of Cretaceous brachiosaur, since the show has depicted a number of animals not found in the fossil record like Acceraptor, Ovosaurus, Malcolmus pterosauri, and Nykoraptor.
In the first episode, the brachiosaur is stated to supplement its herbivorous diet with lizards, something there is no evidence for although there aren't many fossils of brachiosaur stomach contents or droppings (a discussion of fossils relating to sauropod diet can be found here).
In the shot from Proof below, a larger brachiosaur with red coloration on its neck is seen with its neck resting against a smaller brachiosaur with no red coloration. This might be a breeding pair, in which case the red coloration and larger size might be characteristic of males.