Biggest reason is, because it is a RP community, based on RL, What i got, is an "Alarm Company" and so people in the are will request a system, its not like the Orbs. It is just like a RL one, with sensors etc. People are gonna wanna use the doors they already have, assuming they are mod rights, and just attatch the sensor to it. That way if the door itself opens, say the front door, It will send a message on a private channel to the Control panel and let the system know, Door is open, not ready, when the door is closed, the system would reflect, Ready.. etc. that is the reason why it does have to be a prim, not just a script to drop in the door. It does not feel the same, as actually adding a sensor to it, and lots of people have requested me to make one with door/window sensors. It might sound lame, but it is still cool i think. Security, at a different type of level.