While everyone is saying ease of install, and its true, its not really that hard to hardwire a system in most houses. In fact, my previous house was hardwired by a central security group contractor in about 3 hours for nothing more than a 3 year monitoring contract (@~$20 a month, 6 years ago). Sure I didn't get sensors in all my windows, but I did get them in the doors, along with motion sensors that covered 90% of the square footage of the house, monitored smoke detectors, garage door controllers, and a bunch of other crap.

Probably close to $500 worth of DSC hardware.

Alarm monitoring is like printing money. The fact that ADT charges as much as they do, and installs such a craptasic system has always amazed me.