Firstly because it caused test failures on my workstation, and secondly because if a user does require it, they can supply a config file containing this option.

I hit an issue because my workstation has multiple IP addresses (thanks to VMware Fusion adding two new interfaces with IP addresses). When MongoDB resolved SoftwareProcess.HOSTNAME it got one IP address and bound just to that IP. When the Brooklyn resolved SoftwareProcess.HOSTNAME to find out where to connect the driver for its sensor readings, it got a different IP address, which MongoDB was not listening to. By omitting the config option, MongoDB will listen on all available interfaces.