Generally, there are four different reasons stages didn't come back in Ultimate: incompatibility with 8 player Smash, incompatibility with Stage Morph, broken mechanics outside of the source game, or assets specifically ported over from another game.
I'd argue that Orbital Gate Assault fit three of these. Mechanically, it was generally considered one of the least popular stages in Smash for Wii U, so that didn't help it. But I also think it was one of the stages that took up a LOT of resources to run effectively. If I remember correctly, it was one of the stages in Smash 4 that occasionally struggled to run in a more chaotic four person match. I imagine 8 players would only exacerbate that problem, especially with a character like the Ice Climbers in play. I just don't think the engine could handle the strain of the stage in addition to everything that Ultimate was promising. The original stage also probably didn't have the kind of payoff they had hoped for all the effort invested, so I'm sure that also made them not super keen to put the effort into making it return or finding workarounds.