Diesels aren't inherently more maintainable than gasoline engines. That impression comes from the fact that diesel engines are used in situations where durability is paramount, thus are designed to be robust. Most passenger car engines have several criteria that are more important than maintainability (emissions, power, fuel economy, size, cost, manufacturing simplicity).

People don't hold up BMW passenger diesels as the hallmark of maintainability. Heck, even the diesel offerings in American trucks are pretty bad from a reliability standpoint. Both Cummins and Navistar faced multiple CALs over reliability and warranty-related issues.