I had a thought about exactly why no Lunars have ascended to Cthulhu-hood yet, despite having almost 5 alternate-calendar millennia to do the job in: It's the moonsilver tattoos that make it possible.
I envision the tattoos acting like the control rods in a nuclear reactor, safely preventing China Syndrome--which in this analogy means Chimerahood. The problem is, the rods are jammed in all the way. A catastrophic meltdown is completely and utterly impossible, but the tradeoff is that you can't get the reactor running. But with an operator that has enough training, you can take the rods out further and further before you start risking a meltdown.
You recall how in MoEP: Lunars, there are some Lunars who are convinced that the way to true power is by rejecting the Caste-fixing tattoos? They're all wrong, and the reason their experiments keep ending in disaster is because they're trying to fire up the reactor without any control rods whatsoever. Their predictable failures have spooked the other Lunars so badly that nobody's willing to shake up the status quo--and honestly, who can blame them?