[whpalmer4]

That reminders calendar was implemented as a way to provide alerts to iPhones or iPods that might not have Internet connectivity that something was about to be due, even if OmniFocus wasn't running. Apple didn't provide a notification service that could be used by developers to do this before iOS 4. By putting pending due events in a special calendar, Omni gets the Calendar app to do the dirty work.

As for why it only goes 14 days out, that seems like a reasonable timeframe likely to cover most people's needs for populating that calendar between syncs, and the calendar needs to be rewritten each time you sync if there are any changes to due dates in that timeframe, so there's a cost to be paid for extending the timeframe too far. It's sort of a happy accident that Apple didn't provide an API for notifications and that Omni had this idea for a workaround; had an API been provided, there probably wouldn't be any way provided to see the notifications in advance as you can with the calendar trick.

You should send in feedback with Help->Send Feedback if you want an option to control the size of the calendar; if enough people are interested in such a feature, it might get more attention than it would otherwise get as a happy side effect of something that is needed by a diminishing group of users.