DisplayPort to HDMI/DVI/VGA conversion is deceptively complex because the two standards use different electrical signaling. DisplayPort comes in two versions—"Dual Mode" DisplayPort (DP++), which allows the port to actually switch its electrical signaling to a different format, and "pure" DisplayPort (DP), which is intended for DisplayPort monitors without any conversion.
Passive adapters rely entirely upon DP++ functionality to produce the appropriate output signal. In other words, passive adapters do not work when connected to a regular DisplayPort port (only supported through a DP++ port).
Active adapters on the other hand contain extra components that perform the necessary signaling conversion on their own, so they do not have the same reliance on a DP++ port as a passive adapter would. In other words, active adapters are more flexible and can be used on either a pure DisplayPort output or a DP++ port. To hit the point home, passive adapters only work through DP++ ports.