Some issues were with the specificity of DOT regulations for "passive restraint" systems. VW, for example, had airbags on europeans models that were too small for the DOT-required minimum so US-model VW's had the ridiculous "auto-seatbelt" that was connected to the door. IIRC, the requirements were some sort of auto-seatbelt or an airbag that met DOT minimums.

Remember the motorized the shoulder belt?