most power supplies do their sensing and regulation on the high side of the load. open-frame power supplies usually have sense lines to measure the voltage at the load. the current measurement for the current limiting is done across what's usually a 0.1ohm or 0.001 ohm emitter resistor for the series pass transistor. the voltage and current sense lines feed a pair of op amps, one measures the voltage at the load, the other measures the current. these op amps control the series pass transistor. both the output voltage and current can be set with potentiometers that change the reference voltages of the op amps. this is all done on the high side of the load, not the low side, because everything knows where "ground" is, but your circuit doesn't know where the supply voltage is, and doesn't sense it at all. some things that regulators do is actively filter out noise. you can't do that on the low side of the load, because you can't see what noise or other variations in the supply voltage may exist. even if you did, those variations and noise exist on the high side of the load, and that's where you remove them, rather than the low side, where all you're accomplishing is to make the load float from ground.