Why doesn't this diode conduct in a boost converter?
Current only flows when there is a potential difference (voltage) between two nodes. Assuming this is an "ideal circuit", the switch in your schematic has zero resistance, and therefore the anode of the diode is at 0V (assuming the bottom line is the circuit "ground" reference). The cathode of the diode will either be at 0V or higher, and in either case the diode will not conduct.