It could be as simple as the EPA Tier 4 regulations concerning the DEF refill interval requirements. "For light-duty vehicles and light-duty trucks equipped with SCR aftertreatment, a minimum DEF (urea solution) refill interval is 4,000 miles."
"For light-duty vehicles and light-duty trucks (LDVs and LDTs), we are adopting a minimum interval of 4,000 miles. Under the (b)(7) process, we typically had been requiring DEF refill intervals at least equal to the scheduled oil change interval for the vehicle, which is typically more than 4,000 miles. Thus, for LDVs and LDTs, the final regulations differ in two ways from the previous policy: The DEF refill interval is being
decoupled from the oil change interval, and the minimum interval is being shortened."
"To put this 4,000 mile interval in context, a vehicle with a 400 mile fuel range would need to refill the DEF tank no more frequently than every tenth fuel fill up. For operators who change oil every 7,500 miles and fill the DEF tank when they do, no more than one DEF refill would be needed between oil changes. We still believe it is necessary to require substantially longer DEF intervals for LDVs and LDTs than for commercial heavy-duty vehicles because of the wider range of usage patterns of light-duty vehicles. Most significantly, these light-duty vehicles are more likely to refuel at neighborhood refueling stations that may not have DEF. Ensuring that these vehicles can go through several tanks of fuel before needing to refill the DEF tanks reduces the likelihood that operators will allow the DEF tank to become completely empty."