Rusty Kick Panels

[1969 GMC]

I haven't really seen this mentioned on here alot, but my GMC doesn't have any rust in the floorboards...its rusting through in both of the kickpanels. By where the oval openings for the fresh air vents are, to be exact. The passenger side is really bad...quite a bit of rust thru. The rust is only on the inside, there isn't any on the outside. When it rains this side leaks and gets my floorboards nice n wet (but they aren't rusty). The drivers side only has rust thru in a tiny (pencil tip sized dot), it doesn't leak. When I fully open both fresh air vents, there is a ton of leaves, maple helicopters, and these damned dried up berries inside. If I open these at speed, it's like someone threw a bag of confetti inside the cab.

My questions are:
1). How did this happen? Why don't I hear alot about these rusting?
2). How can I fix? Are there patch panels or do I have to cut up a parts truck?
3). How can I prevent future incidences of this, and perhaps save my drivers side?

thanks for your time