Their administrative structures were similar, there was a similar link between secular and religious power, albeit different religions. All of which made it relatively easy for the Spanish crown to step into the void left by those two great empires and send their own administrators to run the place. While most of the Spanish aristocrats who came over ran large agricultural operations, you don't see a lot of movies called, like, Indiana Jones and the Search for a Nice Farm in the Countryside. The real glory for conquistadors was gold. Initially, they found some, both in the Caribbean and in Mexico, but never enough to get, like, super rich. Fortunately, or as I will argue, unfortunately, they did find a mountain made of silver. So the Spanish adopted the mita, which the Incas had used to build roads and public buildings, to mine and process that silver.