Because it's cheaper to produce. And that may not be so bad since it's cheaper for you as well, to maintain at least.
They are also smaller in dia (not width necessarily) for the same braking force when compared to discs. This is due to the fact that the entire surface of the brake pads press against (and at) the circumference (in an axial direction). If you have alloy wheels and brakes are visible, then they might look a tad bit tiny compared to a similar class car employing disc brakes. So , if you're looking for street creds.. painting them red may not really..
Since the pads press against the drum shoe in an axial direction, they have a tendency to stick on/force the pads into full contact. This effect reduces the need for brake assist mechanisms, especially on smaller cars where it can be done away with (again, a plus for the manufacturer). Consequently, this effect, which affects the response time, could possibly be one reason why their use with ABS and similar systems is rare.
Drum brakes do not dissipate heat as well as disc brakes (open and exposed) which quite simply is its biggest disadvantage over disc brakes. This precludes its use from performance applications, though, there are exceptions.
As other answers have pointed out, this may not necessarily effect you in emergency braking situations as most of the weight shifts towards the front of the vehicle and hence most of the braking is done by the front brakes (discs in your case). This, of course is assuming that the car is pointed in the direction of travel. High speed pursuits/get-aways in reverse gear might require a different setup (fast and the furious. anyone?)
Some clarifications -
Drum brakes do, in fact, have more moving parts than disc brakes, and do have a more complicated mechanism .
Can be adapted easily for mechanical/cable linkages , making their use popular for parking/emergency brakes