Because you don't need IIS
One of the often repeated, and frankly, somewhat unfair, arguments against using .NET based technologies is that by doing so, you handcuff yourself to all kinds of various Microsoft tools and products.
If you choose to use ASP.NET Web API, by no means you are tied to IIS. Of course, one of the options is to self host it (and run the web service using the hardened WCF core), but if you want a true web host flexibility and independence, you can very easily host Web API using OWIN and Katana.
More after the jump.