Nothing prevented them; Detroit automakers either had substantial interests in foreign makers selling light trucks outside North America, or were in some cases

Nothing prevented them; Detroit automakers either had substantial interests in foreign makers selling light trucks outside North America, or were in some cases selling small pickups and vans designed by their own foreign divisions. They simply did not feel that smaller/lighter trucks & vans would be profitable, or profitable enough. This goes off and on; often one of the Detroit makes will go years without having an entry in the small truck market. There are advantages to using a pickup-style perimeter frame to make a van, that said, unlike Ford & Chevy, the Dodge B-series vans were unitized from the '70s on.