The biggest problem is the size of the maps/software and updating. The mobile navigation industry is taking off in a big way and in order for navigation software to stay competitive they have to be able to offer more then just point to point navigation.
Once you start throwing in business searches, hybrid/satellite maps, etc it just isn't practical to try and cram it all on a Micro SD card. It "could" be done (and for some mobile platforms it already has been) but then when you want to update a street or business address you either have to connect up via a PC and update the lot (PITA) or update as you go via your phones data connection (might as well just have it all on demand from the data connection).
Also you then have the issue that a lot of people now use their phone memory (and memory cards) for music, photos, etc and swapping SD cards around in a BlackBerry is a PITA as well.
Until BlackBerry's have a much larger memory capacity or a way to quick switch Micro SD cards I don't see many developers going for the "offline" option. One other thing to consider as well is that a lot of the halfway decent "pay once" packages actually cost more then then subscription ones (even taking data charges into consideration).