Long story short: the FS devs wanted a different kind of freedom than that which is offered by the GPL. The GPL is "viral" in that it forces everything it touches to by GPL-compatible. This kind of force-fed freedom doesn't always fit well with software that is technically a library (libfreeswitch) that can be compiled/embedded in other projects.

Bottom line: no OSS license can be all things to all projects. Tony chose the best one available for his specific project.
-MC