The Kings had in fact brought wealth to Rome. The Patricians were happy as they got a cut of the booty. The Plebeians on the other hand were increasingly unhappy: they were forced to join in the fighting of wars but they got no share of the wealth. Furthermore, the plebeians tended to be poor and this meant that time spent out at war was time spent away from work and hence a need to borrow and be subjected to horrific usury and even slavery.
The rich became richer and the poor became poorer and whilst in the early days the claim to Roman Citizenship of the Patricians seemed well justified this right grew thinner and thinner as many generations later the population, size of the city-state and political arena of Rome had completely changed.
The first two centuries of the Republic were characterised by a paradoxical situation of social unrest at home which was accentuated by continued military conquest abroad, first across Italy and then Carthage and the Mediterranean trade routes.
Military conquest quickly followed by expansion eastward when the Greek cities called for Roman aid against enemies such as the Macedonians. Clearly the Romans hoped for nothing less and shortly achieved dominion not only of Greece but also of Asia Minor. Egypt fell under Roman control by 30BC.
The enormous territorial expansion and the subsequent influx of wealth (and slaves) to the city fed the need for a re-adjustment in social structure. Rome and indeed the whole of Italy were shaken by violent political and social unrest. The Plebeians grew tired and hungry and threatened to leave Rome, the Patricians worried and gave up morsels of power. The least they could give up the better. However little by little the plebeians gained ground.