For the same reason that you put a certain type of gas in your car. Everything in the electronics world has a given specification that it operates efficiently at. Given users such as yourself, who have no clue on the wiring, impedance load, current draw and actual output, it's no wonder manufacturers suggest that professional installation is recommended.
EDIT: I know my above comment is gonna make somebody's sig. //content.invisioncic.com/y282845/emoticons/rotflol.gif.b453361716769b8110ddefc85ff03cd2.gif