The physical factor that facilitated the Spanish colonization of Mexico was:
The Aztecs died in large numbers from diseases introduced by the Europeans.
When the Spanish arrived in the early 16th century, they brought with them diseases such as smallpox, which devastated the indigenous populations who had no immunity to these foreign illnesses. This significant decline in the Aztec population weakened their society and made it easier for the Spanish to conquer and colonize the region.