This is almost assuredly due to the lack of a sound institutional structure which is needed for a market economy to develop. An invading country may be able to bring this structure to Somalia.

It's worth noting that before Ethiopia invaded with tacit US permission*, Somalia was being incrementally conquered and pieced back together by the Islamic Courts Union. The impression of them I got early on, before the US media began to consider them dangerous for being Islamic and fell in line behind the Ethiopian invasion, is that they did a pretty good job of governing - and certainly a better job than either the warlords or the government the Ethiopian installed.

• Arguably a major fuckup by both parties