Opportunity cost, perhaps? Trucks are an easy win if you've got launchable performance in cities, but still require a lot of human energy to get into production.

So early on, when the number of people that know the system is small, it seems better to work on the bigger, more general problem, and branch out to the sub problem later on when there's more people to work on it.

From another perspective, of you stop work on cities to finish a trucks launch, you're losing ground to competitors working on cities, which may be decisive in the research phase.