Mostly because I like the geofencing aspect of Reminders better than the OF implementation, and because I'm trying to find a good integrated calendar/to-do app, and virtually all of those use Reminders as a source for tasks. I've been considering switching from the (awful) native Calendar.app to Pocket Informant, because I really do like seeing my schedule and tasks on the same screen.
The Forecast mode doesn't really help with that (due to lack of options and filters, and a generally pretty but not very functional presentation), and more importantly, it's not the default view when I bring up OF on my iPhone. As far as I know, there isn't any quick shortcut to change to that view. That would be an excellent feature to add, by the way -- allow the user to assign gestures to jump to different views.
Basically, I think OF is a good option for capturing and organizing tasks, but the mobile implementation really doesn't work into my workflow at all.