I can also help address some of the other parts of your question, which I think get to why these aren't necessarily very well known.

So when you say "these Pyramids remain unguarded, with no sign of tourism." Well, there's a reason for that. Since 1993 Sudan has been US designated state sponsor of terrorism. This followed still-President Omar al-Bashir's military coup d'etat in 1989 and his subsequent transformation of Sudan into an Islamist state. Sudan was, among other things, Osama bin Laden's home from 1991 to 1996.

The Khartoum government has also long been engaged in civil conflict with various regions and peoples of the south. The Second Sudanese Civil War ran from 1983-2005 and eventually resulted in South Sudan's independence in 2011. The Darfurs and other Sudanese states remain active areas of conflict.

This has had a variety of effects on tourism and knowledge of Sudan. While the north of the country where these pyramids are has been less affected by violence than the south, the country as a whole suffers from an incredibly negative international reputation. As a result of the state-sponsor of terrorism designation, Sudan isn't part of the global banking system. For example, it's not possible to withdraw cash at an ATM in Sudan.

Sudan's government is also deeply repressive. Even in contrast with other regional autocracies like Egypt, the news media in Sudan is extremely limited. It also restricts visa access for arrivals to Khartoum as well requiring movement passes for foreigners to travel in many parts of the country.

The result, as you note, is that unlike neighboring Egypt, where pharaonic tourism is a major attraction, Sudan has no significant tourism industry to speak of.

It's also probably worth mentioning that these 200 pyramids are all very small. So, yes, there may be 200, but many are only about 50 feet tall. While they're certainly impressive, it's not exactly surprising that they are overshadowed by the gargantuan pyramids at Giza, which were the tallest buildings in the world for millenia.

I’m not sure who the “they” you are referring to here is, but I’m going to assume you mean the general US Public Education system K-12 as you certainly can learn about Kush in college (insert marijuana joke). Now Nubia and the Kingdoms of Kush are certainly mentioned in the more advanced global history courses US high school students take (generally in the 10th grade under the name Advanced Placement World History or AP World as the class is more generally called).

So what your general question seems to be is “why do we focus more on some civilizations over others.” Now that is an interesting question with a twofold answer:

1.) History teachers have approximately 8 months (between Labor day and the AP exam in May) to teach over 6000 years of human history. So there is a serious time constraint and, as such, certain - more globally impactful - civilizations are focused on while others are simply mentioned. Why use an extremely limited day to discuss Nubia when events in Egypt and the Greater Fertile Crescent have had a much greater impact on the course of global history? The same thing you say about Kush could be said about the Gauls, the Phoenicians of Carthage, the vast history of the many different Kingdoms of India, or even the many meso-American civilizations. All of these and countless other civilizations have to be glossed over in order to focus on the broader strokes that make up global history.

This is in no way intended to insult or undermine the achievements of the Kingdom of Kush, but I find it difficult to argue that these achievements are of more pressing concern then those of other contemporary regional centers such as Egypt, Greece, Assyria, Achaemenid Persia, or Rome.

2.) It is a matter of sources. Particularly when we’re dealing with ancient civilizations, sources are a precious resource. Peoples who had developed and sophisticated writing systems will inherently get greater focus in general education. As well, we have to be able to translate these sources for them to be of greater value.

Take for example the Indus River Valley Civilization. An early incredibly urban and developed civilization with a population of approx 5 million that surpasses both Mesopotamia and Egypt in scope and size. Yet it remains tertiary to Mesopotamia and Egypt in general history education due to the fact that we can’t understand the Harappan language in which their sources are written.

When it come to Kush, many of the sources we have come from outsiders such as Roman writers or from peoples who may have a serious bias against them (such as conquered Egyptians).