Why do scholars know so little about the site of Machu Picchu?
The reason that scholars know very little about Machu Picchu is that there are no written records about the site. The Incas had no system of writing and Machu Picchu was not known to the Spanish so they could not write anything about it.
When there is no written evidence about a site, scholars have to do the best they can to infer things about the site from whatever evidence can be found. Since this evidence is not really clear, scholars can interpret it however they want. Therefore, there are many different theories about Machu Picchu. Some people believe that it was a religious site while others think it was a governmental center. Some even argue that it was a prison or an agricultural experiment site. This is what happens when there is no written evidence about a site and scholars have to simply make the best guesses they can based on whatever evidence is available.