The simplistic answer to this question is evolutionary adaptation to their environment. The ears of rabbits serve two different purposes. First, they assist rabbits in hearing threats at different angles. In fact, a rabbit's ears can rotate 270 degrees. This helps the rabbit detect and locate threats.
The second function is to help regulate body temperature. Rabbit ears have a network blood vessels that allows heat exchange. For example, the blood vessels will contract when the rabbit is cool, and the vessel will swell up when the rabbit is warm.
From an evolutionary biology standpoint, these mechanisms developed over a course of 4,000 years, and thus physiologically changed the shape and size of modern-day rabbits. It is similar to how our ancestors—specifically Homo habilis—developed opposable thumbs in order to grip objects better. If such a need did not exist, we wouldn't have opposable thumbs today.