Because PoE means:
1) A thick Ethernet cable, which may not be easy to lay especially in older buildings
2) A more expensive switch, or PoE injectors placed somewhere.
Last time I replaced the doorbell system I chose a system able to work on two wires of the older system - including power. I don't have HD video and audio, but I guess I won't see Taylor Swift performing in front of it anytime soon...
Biting the hand that feeds IT
