Why doesn't AirDrop work on older network adapters?
Basically what AirDrop does is it turns your Mac into a WiFi access point in the 5 GHz band. That's why it's zero configuration, works whether or not the Macs are in the same wireless network, or in ANY wireless network.

However, to create this second network, the adapter needs to support multiple networks.

This also explains why it only works with WiFi and not with e.g. one Mac wired into a base station and the second Mac using WiFi to connect to it.