Compared to English colonists in North America, the Spanish Colonies were mostly comprised of single men without wives; Spanish women didn't come to the New World in as large of numbers compared to English women, leading Spanish men to intermarry with the native women. I believe the same thing occurred to many French settlers in modern day Québec. This probably isnt the only reason but my textbook stressed this point a lot. I'm currently in an AP European history course and we use A History of Western Society since 1300 published by Bedford St. Martin's.
hi! other responses are welcome, but fyi, you can get started in this section of the FAQ
Interestingly, during one of the many dictatorships in Paraguay, the leader actually encouraged racial mixing between the indigenous peoples and the Spanish speaking colonists. This was an attempt to create harmonious and racially unique state and people. As a result, there is a very large portion of Paraguay that can trace their ancestry and customs to the indigenous tribes. So, in this case, there was a conscious effort to create a unique Paraguayan people, different from anywhere else.
This rule doesn't hold true in all cases. The biggest thing is that there were two very densely populated areas in the New World, Mesoamerica and the Andes. These people were able to hold together through population decline (from disease, war, etc.) much better than the rest of Native America. The Spanish arrived in these places just as the natives first encountered diseases and kept existing structures intact.
The English, French, Dutch, etc. arrived in eastern North America a full hundred years later and did not find massive cities they could conquer. This is partially because they were already hit by disease and partially because their agricultural systems weren't developed as intensive. The French did interbreed with the natives they found on a large scale like you're expecting.
On the other hand, places in Latin America saw minimal interbreeding because of conditions more similar to what we imagine in North America, for instance around Rio de la Plata. It's important to note that the Europeans were all really more similar than they were different, but they encountered vastly different native societies and environments at different times.