A lot of people don't want a 1 ton duelly for a daily driver.
It seems a lot of people use the more storage in a 5er argument. I don't bring enough extra ((stuff)) along to need more storage.
You can go more places with a small trailer then a 5th wheel.
Cost, allows new families to get into RV's and use the family SUV or half ton to tow with.
A 5er has a couple advantages though, available in high end units, shorter then same size TT, tows better.
Although I have a 35' 5th wheel I use my TT more because it will go where I like to camp.