Mary Beard makes an excellent case in her book SPQR that the contradiction between Imperial rule in the provinces and republican rule at home made some kind of drastic rework of the republic, and probably some form of autocratic rule as developed. In essence you will never be able to reasonably have a system function where outside the bounds of the capital, military leaders and governors can act as absolute autocrats and institutionally essentially take the place of old sitting monarchs and leaders, while at the same time there is at home a pretense of legal equality and rule. The experience of massive campaigns of conquest laid the foundations for a caste of oligarchs with the power and potential to overshadow everyone else within the republic.

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Alongside that it’s worth saying that the republic outgrew the point where it could reasonably govern what it had conquered effectively. The institutions of the republic were in essence not too drastically changed from the days when it was simply a single italian city state that happened to be dominant over its region. But in the intervening time the state had grown to be the dominant one in the known world. In the end no one was willing to consider the kinds of radical reforms that would be needed in order to transition the Roman republic into a republican government capable of ruling the Mediterranean.