Why in most dual licensed software is the GPL one of the licenses?
The GPL is particularly common in dual licenses because it allows the code to link with the large body of GPLed code available including many important libraries, and to be incorporated into other GPLed works. Many common works are under dual GPL/xxx licenses: perl is under GPL/Artistic, Qt is under GPL/QPL, Mozilla is under GPL/MPL. This is almost always due to a project starting with a home-brewed GPL-incompatible license, then realizing they’d made a mistake and finding relicensing in this fashion to be the most convenient resolution.
