Christopher's analysis is just about right. SyFy doesn't cancel programs any more than any other major network, be they broadcast or cable.
The only thing I would add is that the network's production schedule seems to be one that doesn't allow for serialized shows (even ones that have been on the air for several years) to wrap up their stories with much satisfaction. Witness the cliffhanger cancellations of Sliders, Stargate: Universe, Farscape, and now Alphas as well as the haphazard eleventh hour conclusions of Stargate: SG-1, Stargate: Atlantis, Caprica, and Eureka. That kind of treatment makes cultivating a committed fan base a bit hard, I think.
(Although in writing that, I'm not sure it's really all that different from any other channel.)
Indeed. This tends to happen to shows on other networks as well. Look at, for example, THE 4400 and THE DEAD ZONE on USA. Although, again, both of those shows ran for four seasons or so, which is not a bad run, both today and in the past!