This is made special this way. Let me try to explain my thoughts about this:
Most response objects the framework will send, are PhpEnvironmentResponses, so this one has the highest priority. However the priority is low, so if you attach a custom response listener without priority, this one will be used first.
Second is the ConsoleResponseSender, which will obviously only be used for console responses, but I would expect having a couple of them in an zf app.
Last the (Simple) Sream response sender, which will (in most apps) not used, because not every application delivers stream responses to the end user. So as the stream response sender is not important in most apps, it has the lowest priority.
The reason why I chose different levels of priorities is, that you can attach a custom response sender with higher or lower priority respective to whatver base response sender is available.