Freemarker is specifically designed to handle the kinds of tasks that a Web Script view (or Space template, etc.) is responsible for ie. producing some kind of text based output based on a data model of some kind. While JSP can accomplish the same things, it's far more general than that (JSPs are really just Java classes after all), and as a result is not as specialised for these tasks as Freemarker.
There are also some practical considerations - Freemarker templates are interpreted, so hot redeployment etc. is trivial. The same is not true of JSPs, since they're compiled down to classes, and hence run afoul of the various class reloading issues in current generations of the JVM.
Cheers,
Peter