what makes you say that? I use bitcoin to conduct private transactions regularly and its pretty darn solid for that purpose. Doing such a transaction over the internet, before bitcoin, required you to use permissioned rails.
Just because it can be used like that doesn't mean it actually succeeded. The vast majority of Bitcoin is held as a speculative investment, and the deflationary aspects are a strong disincentive to regular circulation.
A monetary system that only works if a small number of people use it is...not useful.