devtrackers.gg
I can only speak for when I was Game Director and we implemented ascension paths but the lore reason for no psi ascension then is that HMs are already psionic but in a different and incompatible way (this is why we made sure they can also talk to the Prethoryn for example). More practically both these ascension paths would have required extensive modification for HMs to the point where it'd have been better just to give them their own ascension paths. With machine ascension you also get major overlap with Machine Empires on top of all that.