Because Tamiflu is in tablet form and Relenza is in powder form. And also because Tamiflu was first to hit the markets. But I think the main reason is the advantages that tablet form has over powder. Powders have drawbacks to it, e.g. elderley, small children, asthmatics have difficulty using the drug whereas there's no such problem with a tablet. See for e.g.
http://news.bbc.co.uk/1/hi/health/1209173.stm
