When I bought my F250 in 2005, I needed something to tow a bumper pull toy hauler. There was no advantage to getting a SRW F350 since almost all the components were shared on my truck and the tow capacity was the same. At the time, I compared the SRW F350, Excursion and F250 for my needs and found the F250 to be the best value with no degradation for my intended use.
When I bought my truck camper in 2012, I could have benefited from the F350's 2" taller rear axle blocks and a long bed. However, I didn't think I was ever going to get a truck camper or fifth wheel at that time plus I still would have been limited to the same actual usable payload since the components were almost identical.
My current truck is a 5500. We are looking truck campers that weigh 3 tons and plan on towing over 4 tons behind us so we need the payload capacity. I could ask the same question anyone that bought F350 DRW why they didn't just buy a F550 since it is the same size and payload is far superior. The truth is that people will buy what they need or want and each choice has advantages and disadvantages.
BTW: Here are some advantages to the 3/4t trucks -
1. Less licensing restrictions if used commercially
2. Less vehicle licensing if your state charges by GVWR
3. Less initial purchase cost
4. Allowed in the most restrictive communities where a 1t is viewed as commercial
5. The Ford model sits lower and clears a 7' tall garage opening
6. The GM model is available in short bed extended cab
Host Mammoth 11.5 on Ram 5500 HD
