The reason why it's more fun is simple. While it is of course very hard to write a good screenplay, it's much easier to write a screenplay than it is to write a novel; just considering the sheer blank space on every page of every screenplay, you can understand why being forced to write a little more than three pages of screenplay a day is less work than three pages of solid text a day. Plus, screenwriting is only a tiny piece of moviemaking, and so you can mentally shove the deficiencies on the page further down the process—"surely," you can tell yourself as you write, "the right actor/director/cinematographer/best boy can make this script work somehow!"
Another Script Frenzy perk is that you can collaborate with another writer on your screenplay, making it a much more social endeavor than NaNoWriMo. Plus, there's a broader range of media involved: According to the rules, "You may write screenplays, stage plays, TV shows, short films, comic book and graphic novel scripts, adaptations of novels, or any other type of script your heart desires."
My favorite type of free scriptwriting software is Celtx, which enables you to write just about any kind of script easily. But if you're collaborating, you might want to try Scripped, which is kind of a Google Docs for screenplays. You have a month to plan your outline. Go!