Yep, simple title, "Dirt!"
Some time ago I was able to get the last bit of carpet removed from my fifth wheel. Everything is now linoleum or laminate. Every camper we've had in the past has had carpeting.
It wasn't until we got the last bit of carpeting out of our Montana that I realized how horrid horrible the carpets were (after only 4 years, bought brand new).
Sweeping a non-carpeted floor is so much easier than dragging around a bulky vacuum sweeper. I've commented to may wife many times how much dirt gets swept up with a broom and dust pan now. All that "dirt" had no where else to go except get trapped in carpet, even with the best of vacuum sweepers.
We are currently parked at home. We sleep in the camper every night. It's parked on asphalt. We have a concrete side walk from the house to the drive way. Yet, every 2 days we have to sweep the floors of the camper, and every 2 days we get a pile something like this in the photo below I took just this morning. Imagine all that getting trapped in a carpet. No wonder after a short while the carpets were misserable. I can't imagine how nasty they got in our previous campers.