To keep it alive for a little longer, at cost. Greenlighting one television movie is far cheaper than greenlighting the equivalent of ten times that in the form of another season (2 hours vs. 20). Plus, it can be advertised as an "event," which has been successful for SyFy in the past. Why they are dumping it well after the longer DVD version has been released with little media push is...well, to be honest, all too typical of the network's idiocy.
My point about Caprica remains the same. A "new" series, even one that is a spin-off of a previous series (and thus, will continue the brand for a little while longer) is both cheaper to produce and more liable to attract new viewers than a fifth season of Battlestar Galactica. Even the format, which is planetbound and completely lacking of space travel in the pilot, suggests thoughtful cost-control.