The answer is...we just don't have enough information yet.
If the Clan Toumans shrunk in the years between the end of the Jihad and the beginning of the "Dark Age", then the Clans are probably sitting on an excess of mothballed droppers (for military units that no longer exist) and find it preferable to break out the dropships sitting in mothballs or being used for cargo runs as vehicle droppers.
Also, plenty of years have passed, the other Clans could have picked up the Outpost design and started building it. With a little tinkering, the room for that mech star could become additional vehicle space.
I think variations of the Outpost, and modified versions of the Union and Overlord are the most likely scenarios here. Another plausible scenario is the Clans resume construction on an IS vehicle dropper like the Triumph or Gazelle, or just throw captured ones into service.
A build-from-scratch vehicle dropper doesn't seem necessary to me. There are cheaper alternatives. Less efficient, sure, but the Clans have never put a premium on their vehicles and catered to their needs very well, they get the low-end of the budget.
But again, so much has changed during and after the Jihad, and we're still learning about it.