Arguably, sieges have simply changed not gone away.  For example, in the Second World War you could consider the German blockade of Britain an attempted siege, as well as Malta, and the Allied blockade of Japan. 

In a Traveller setting I imagine whole planets under siege, rather than castles or cities. Also, with space stations you might want to capture it mostly intact rather than simply obliterating it and the defenders, so I can see some sort of siege there. Creating a breach, attempting to storm the orbital city, neither side wanting whole-scale destruction of the asset or its inhabitants.