It's just the bed construction. Ram has a perfect little spot for stuff to gather in there and even filled with foam so it's harder to rinse out. Made somewhat worse by the lack of fender liners. Rinsed and treated properly each year, not an issue.
They all have their areas of rot, none are perfect.
Another bad spot on the 4th gen, in particular, is the presence of an inner and outer rocker cavity. I drive by a house with a 4th gen outside that is rotted clear through the outer rocker and bed side on both sides of the truck. It's likely 12 years old now, but still. A coworker has a '10 that's not too far behind...another two years maybe.
There's a couple different camps for people in the rust belt. Either you learn your lesson by letting a vehicle rot, you complain about them rotting, or you trade and buy new every couple years. I learned my lesson on a truck I really really liked.