What tech is holding back the implementation of the cargo ramps (elevators) on the Drake Caterpillar?

They don't know that there's any tech stopping it. They have the required animations, as well as the Interaction State Machine (what handles the interaction mode), it's that they're missing the ability to trigger the state machine to go into those states required for the elevators to work. The reason they're not yet functioning in-game is likely because it's something in their backlog that they haven't gotten to yet. They will get to it eventually, they just need to sync a programmer and a designer to do it. Incidentally, it might get looked at next quarter by the Active Feature Team who will be looking at the Player Interaction System in Q3 of this year.