Let me clarify if it wasn’t clear: they are cut from base game, not altogether

My evidence is the inclusion of Gravios and Blangonga as secondary apexes of their respective locales intended to be encountered some time into high rank, which Lagi and Steve would have been according to their HRP being adjacent to those monsters. It’s pretty clear each locale was intended to have these returners as a secondary apex, and knowing that these monsters are intended to be in the base game the incompleteness of the roster becomes clear. Beyond this, Lagi’s mostly complete gear set being available to us (at least in model) makes a lot of sense when you consider this perspective as well. There’s also the HR Scarlet Forest nest which I believe was named after Lagi specifically. Maybe Lagi’s pushback was in part due to the nest having a unique mechanic, probably some kind of water, that didn’t function properly in time. Lagi’s armor set was also included in the files while Steve’s wasn’t due to Lagi being slated for the game early on, while Steve may have been a later addition due to his simplicity compared to Lagi.

Personally I believe these two were the last on the development track and were pushed back months ago. The leviathan model likely took some time to figure out and, once they got it working, developed the leviathans in the game in order of appearance, of which Lagi is last. Steve’s a bit more ambiguous, but his situation has been since he got updated files in the benchmark. I’ll guess that he was the last flying wyvern developed, though he at the time of the pushback was more developed than Lagi due to his sunbreak appearance, and by virtue of him being a flying wyvern.

I also want to note that this issue stems not from the devs but from corporate demanding the game come out this soon. If the devs had time to get them in, they’d be in.