Microsoft has written a Knowledge Base article on the subject. The following is a quote from Q244209:

It is a common practice in high-traffic network environments to replace traditional Ethernet network hubs with intelligent switches. While this can improve computer and network performance, it makes tracing network traffic between computers difficult. An Ethernet switch works by directing packets to a specific MAC Address associated with the port. Because of this, when you try to use a network analyzer (such as Microsoft Network Monitor) in this environment, only packets with the analyzer computer's MAC address or broadcasts appear in the trace. This article describes two common methods for working around this behavior.