Windows Azure Web Sites supports two modes, shared and Reserved.
In Shared mode, your web site process (w3wp) runs alongside other, sharing resources including CPU and memory. The runtime (that is Windows Azure Web Sites system) remove sites from memory after period of idleness – that is when your site doesn’t get any traffic. The runtime make the decisions removing sites, bases on many parameters, and even if you change the configuration, it may not apply, as the runtime may override that configuration.
If you are looking for your site to always be in memory, you may want to look into switching to Reserved Instance, in which your site(s) are running on their own VM(s) and the system is much more flexible in terms of resource utilization.
Yochay (PM Azure Web Sites)