Switches aren't included in a traceroute because they do not have an ip address. The switch is just used to pass the packet along to the correct destination. There are switches that you can enable ip address and at that time, it may modify the packet destination ip address.
With that being said, traceroute works on layer 3 and switches can be layer 2 or 3. If the switch is layer 2, it will simply forward the packet. Layer 2 is about networking with the mac address. The switch takes a packet and forwards it to the correct port base on the mac address. It does not look at any ip address.
This question has been answer on another site as well. http://serverfault.com/questions/457796/is-it-possible-to-include-switches-in-traceroute-hops