Because the JungleDisk solution is clunky. You're either going to use Dropbox if you're a casual user or use a Linux filesystem in user space to interface to S3. JungleDisk is the purgatory no one wants to be in.
Note: I've used JungleDisk; my business partner still uses JungleDisk and complains about its shortcomings; I tell him to switch to Dropbox.