The Italian Wars
Political Situation in 1494
Milan
The duchy was ruled at the time by Ludovico Sforza, grandson to the famous mercenary Francesco Sforza. Milan was economically wealthy, but it had suffered from a run of bad luck in its rulers, and Ludovico was neither strong nor politically astute. He was a classic case of a ruler who wanted to run with the big dogs but who lacked the necessary resources and skill.
Venice
La Serenissima was at the end of the 15th century in its last grand days. It was both wealthy and politically strong. It had extensive mainland holdings which brought it additional wealth but which also aroused jealousy among the other cities, especially at Milan and Rome. She had also been recently hard-pressed by the Ottoman Turks and had lost considerably among her holdings in the eastern Mediterranean. So, while she had one eye on the terra ferma she also had one eye on her maritime empire.
Naples
Il Regno was still politically disordered, although she had been fortunate in her rulers recently. The Kingdom of Naples and the Kingdom of Sicily were still separate, though both were ruled by an Aragonese dynasty.
Sicily had been ruled by the house of Aragon since the 13th century. The Aragonese had conquered mainland Naples in 1435. Alfonso V died in 1458, leaving Aragon and Sicily to his brother Juan II, and the kingdom of Naples to his illegitimate son Ferrante.
In 1494, Ferrante II died, and a political explosion occurred.
France
That explosion occurred because the pope invited the King of France into Italy to settle the question of who would rule Naples. Why the French? Because they had claims going all the way back to the 13th century, when a pope had offered the crown of a then-united Sicily and Naples to Charles of Anjou, if only he would help defeated the hated Hohenstaufen. Charles did that, and the Angevins ruled for nearly a century.
In 1494 the French were ready for such an undertaking. The Hundred Years War had been over for forty years. The threat of Burgundy was ended in 1477. King Louis XI would never have got involved in such a quagmire, but Charles VIII was just the right sort of fellow at just the right time.
Elsewhere
What about other players? Then English had no interests in Italy. The Empire did, but it had yielded most of its interests long ago and Maximilian I was not at all the right sort of person to get involved. Besides, the last thing the pope wanted was for the Empire to start meddling again in Italy. Over in Spain, which might have taken an interest, Castile was recently united with Aragon (Isabela and Ferdinand) and had just completed the Reconquest. All their attention was focused on matters in Spain (and on that odd Italian fellow who was just back, claiming he'd sailed to the East Indies).
In short, this was a matter the Italians were free to settle among themselves, with the addition of the French, who turned out to be hugely disruptive.