Well, first of all, only the applications that are included with the CD / ISO can be set as the default. And because there is little overlap between the included applications, there is little choice to be made as to the default.
The real underlying question is "How are the included applications chosen?"
Here is some helpful information from Wikipedia:
At the beginning of a new development cycle, Ubuntu developers from around the world gather to help shape and scope the next release of Ubuntu. The summit is open to the public, but it is not a conference, exhibition or other audience-oriented event. Rather, it is an opportunity for Ubuntu developers, who usually collaborate online, to work together in person on specific tasks.