They don't have the compute to make Mythos generally available: that's all there is to it. The exclusivity is also nice from a marketing pov.
I've read so many conflicting things about Mythos that it's become impossible to make any real assumptions about it. I don't think it's vaporware necessarily, but the whole "we can't release it for safety reasons" feels like the next level of "POC or STFU".
They don't have demand for the price it would require for inference.
They are definitely distilling it into a much smaller model and ~98% as good, like everybody does.