They brought diseases with them which infected the aztec
They had better weapons
Stronger armies and government.
The history of North America is the study of the past, particularly the written record, oral histories, and traditions, passed down from generation to generation on the continent in the Earth's northern hemisphere and (chiefly) western hemisphere.
Pre-Columbian Aztec society was a highly complex and stratified society that developed among the Aztecs of central Mexico in the centuries prior to the Spanish conquest of Mexico, and which was built on the cultural foundations of the larger region of Mesoamerica. Politically, the society was based around the independent city-state, called an altepetl, composed of smaller divisions (calpulli), which were again usually composed of one or more extended kinship groups. Socially, the society depended on a rather strict division between nobles and free commoners, both of which were themselves divided into elaborate hierarchies of social status, responsibilities, and power. Economically the society was dependent on agriculture, and also to a large extent on warfare. Other economically important factors were commerce, long distance and local, and a high degree of trade specialisation.
In the middle of the first millennium CE, the first waves of tribes speaking the forefather language of the Nahuan languages migrated south into Mesoamerica. They were nomadic hunter-gatherers and arrived in a region that was already populated by complex societies at a highly advanced technological level. Under the influence of classic Mesoamerican civilizations such as the Teotihuacanos, the Maya, the Totonacs and the Huastecs the proto --Aztecs became sedentary agriculturalists and achieved the same levels of technology as their neighbouring peoples. They held on to their language, many of their religious systems, and probably aspects of their previous social customs. Resultingly the foundations of "Aztec society" were developed as a synthesis between Mesoamerican societies and Aztec traditions, although today it cannot easily be discerned which parts come from where. Aztec society was not isolated from the larger Mesoamerican context, and in fact, most aspects of it were similar in structure to what existed in the surrounding societies.
The siege of Tenochtitlan, the capital of the Aztec Empire, came about in 1521 through the manipulation of local factions and divisions by Spanish conquistador Hernán Cortés. Though numerous battles were fought between the Aztecs and the Spanish army, which was composed of predominantly indigenous peoples, it was the siege of Tenochtitlan that was the final, decisive battle that led to the downfall of the Aztec civilization and marked the end of the first phase of the Spanish conquest of the Aztec Empire. The conquest of Mexico was part of the Spanish colonization of the Americas.