I'm assuming they wanted to get the dungeons finished first, as they are the easiest to create.

There is a trial one can run with Duty Support at this point. It is called The Mothercrystal.

So, the technology exists, they've tested it out in a major MSQ story point in Endwalker, and I expect SE to start converting 8-person MSQ trials at some point starting in the next expansion.

Duty support for the 4-person ARR MSQ trials are already in-game.