Why is there a pin missing on my 24 pin connector?
This empty pin on your 24 pin connector used to be home to a wire carrying -5V. Since around 2003 the -5V rail was removed from ATX specification and as a result most modern power supplies do not include it.
Most likely it is because the -5V rail has been removed from modern power supplies. If your power supply unit has an empty pin on the 24 pin connector (see picture above), then your unit does not have a -5V rail.
If you're replacing a power supply with a -5V rail most likely you can replace it with a unit that doesn't have one. The -5V rail was dropped as a requirement because there were very few devices that required it; by 2003 it was pretty much just there for legacy ISA cards which were already on their way out of the market.
If for some reason it turns out you absolutely need a power supply that has a -5V rail, the main company we're aware of that still makes those units is Athena Power. There are probably a few others out there as well, so it might be worth searching around.