There's a lot of speculation that it is indeed a marketing plot and the model is just a step improvement over current capabilities... and the real reason they aren't releasing the model is they are compute constrained and cannot serve the model. To my knowledge there's no proof of this however, but given the fact that literally 60 days ago they made Mythos out to be the end of the world and last Friday they announced that they will release the model in a few weeks, I feel like it was indeed something along those lines (marketing ploy).
Or just control of supply and demand. If they can charge twice as much serving half as many customers, that leaves a lot of potential future customers leftover.
The week before they released Mythos to governments they had all their source code stolen. It's all about improving their image and creating propoganda.