If there were ownership in bitcoins, when you received bitcoins from another person, you would be exposed to the risk of someone demanding that you return the bitcoins to them, arguing that they are the owner instead of you – possible reasons for this could be, for example, if the bitcoins were stolen from them or if the agreement under which they sold the bitcoins was nullified.

http://www.coindesk.com/what-tokyos-mt-gox-ruling-means-for-bitcoin-in-japan/