There are any # of answers.
Overall my first answer would be quality and design.
I have had both and the quality of parts inside and out was significantly different.
2 Dodge / Chrsyler minivans the engines lasted a long time the tranny's were very short lived.
2 Ford Aerostars not very many problems at all. The engine is harder to service due to access issues.
Also it could have to do with how the vehicles are driven and maintained. If you beat an Aerostar hard I am sure you could kill it. Yet if both vehicles are treated similarly I am sure that the Aerostar will last much longer. Part of the reason is that the Aerostar is rear wheel drive while the Dodge / Chrysler minivan is front Wheel Drive.
Unless you get Dodge / Chrysler minivan with the 4 cylinder engine and the 3 speed auto or a manual transmission you will certainly learn quickly that the transmission has a very short life span.