Hundreds of books with thousands of words offer all manner of reasons but none quite as concise as these words.
Teddy Roosevelt on the Fall of the Republic
"The Roman Republic fell, not because of the ambition of Caesar or Augustus, but because it had already long ceased to be in any real sense a republic at all.
When the sturdy Roman plebeian, who lived by his own labor, who voted without reward according to his own convictions, and who with his fellows formed in war the terrible Roman legion, had been changed into an idle creature who craved nothing in life save the gratification of a thirst for vapid excitement, who was fed by the state, and who directly or indirectly sold his vote to the highest bidder, then the end of the Republic was at hand, and nothing could save it.
The laws were the same as they had been, but the people behind the laws had changed, and so the laws counted for nothing.”
Rome's decline begin when the people abandoned their Republic in a favor of a Democracy. They exchanged the rule of law for the rule of the majority.
Can you honestly say that is not happening to Americans' constitutional guarantee of a republican form of government?
” They exchanged the rule of law for the rule of the majority. “
No. Not even close. Rome NEVER was a “democracy”. They were a Timocracy that ended being an oligarchy.