The major link in the chain that you're missing here is the breakdown in the elaborate series of checks-and-balances that underpinned the Roman Republic. For most of the Republic, the army consisted of a citizen militia commanded by military leaders whose terms measured months at most, not years. Even if they possessed "extraordinary leadership and military command talents," generals never spent years in the field with their soldiers. As Rome got bigger and wars became further-flung, you begin to see people in charge of armies for much longer which allowed them to build up the ironclad loyalty required to lead an army against Rome. Scipio Africanus is an early example, but this sort of thing became much, much more common in the late Republic.