Hi. It is because Wireshark capture all packets which go to and out your interface while your wireless router select the packets and send them to relevant machines. That mean only the packet to or from your machine can be captured by wireshark unless your network using a hub for connection between other machines.
I have just installed Wireshark on a laptop which I want to
use to monitor my home network. My setup is three desktops connected to a
Westell 327W Verizon DSL wirless router. One desktop is hardwired and the other
two and the laptop are wireless. The hard-wired desktop is using XP Pro SP2 and
all the other desktops and the laptop are XP Home SP2.
When I initiate Wireshark on the laptop it seems to see
everything that is occurring on the laptop but not very much on the other PCs. Why
is that? Am I expecting too much from Wireshark or do I not have it configured