H5N1 An Influenza has not become an epidemic till now. What is the most impoant reason for this?

An influenza refers to a large group of different influenza virus that primarly affect birds. The vast majority of an influenza viruses donot infect humans. However an H5N1 is a strain with pandemic potential, since it might ultimately adapt in to a strain that is contagious among human.As this has not occured it cannot become an epidemic.Founately H5N1 virus doesn&;t jump from birds to humans of spread readily and sustainably among humans. Reference; Park&;s Textbook of preventive and social medicine, 24th edition.Pg no. 166