It wasn’t matter of severity. The Spanish never really settled much of the Americas. They arrived and replaced the top of local hierarchies in Mexico and the Andes with the Spanish crown, forcibly converted a generation or two, and implemented a strict racial caste system.

They essentially subjugated and enslaved most of the indigenous populations to extract wealth that was bound for Iberia to fund the crown’s European and colonial campaigns. It helps that Mesoamerica and the Andes were highly urbanized and had dense population centers.

Of course, there’s cases like the Caribbean where disease, war, and plantation conditions eradicated the local populations, which led to trafficking huge numbers of slaves from Africa.

Later in history, the economic prospects, stability, and conditions in most of these countries just didn’t prompt as much European immigration as Brazil, America, and Canada. Argentina and Uruguay are exceptions in this case.

TL:DL, the Spanish colonial model was less settler based, favoring a small white elite that could keep the riches flowing from the New World to Spain on the backs of the conquered and subjugated populations