Anon, the Phezzani didn't have a military. Their "power" came from skillfully manipulating the Empire and the FPA to keep them locked in a forever war. As long as the Empire and Alliance were focused on one another as their primary adversary, Phezzan would always be an afterthought, which allowed them to move more easily in the background to maintain the balance of power in both the Empire and Alliance.
Nobody ever managed to do what Reinhard did because the Phezzani always managed to deflect interest away from themselves before a powerful leader could focus on them. Phezzan would arrange for some kind of distraction, internal or external, like a rebellion or a major defection from one side to the other, or something else along those lines. But Reinhard's purges of the government and military removed most of the corrupt officials the Phezzani relied on to carry out such acts of subterfuge, and his control over the empire was practically absolute. The Phezzani couldn't infiltrate Reinhard's inner circle, they couldn't foment rebellion because he'd killed, exiled, or deposed anyone in a position to do it, and the FPA was already in turmoil as a result of Reinhard's own destabilization schemes so there was no way to mount a credible threat from Iserlohn that Reinhard would have to divert his interest toward.
They were done. Rubinksy realized this early on which is why he shifted tactics. He couldn't possibly stop Reinhard from occupying Phezzan so he simply disappeared and laid the groundwork for an uprising later.