The paper introducing Bitcoin (https://bitcoin.org/bitcoin.pdf) is well-written and easy to read. Understanding _how_ it works should be simple.
On the other hand, understanding _why_ it works is difficult, as it requires understanding the hard math underlying the system. In fact, nobody knows _why_ it works, because, AFAIK, we do not know whether the hash functions used have the necessary properties (in fact, it is worse: we do not even know whether one-way functions exist, let alone that we can point to one)
While the paper is certainly well-written and reader-friendly, I politely disagree that understanding how it works is simple. Even understanding digital signatures is hardly simple, and that's merely a small subcomponent of the bitcoin system.
Moreover, there's not much point in saying things like that to people who are trying to learn, as it adds nothing except to suggest that if someone doesn't understand this easily just by reading that paper, they might be a bit less capable than some of the rest of us.
The white paper doesn't explain a lot of the very complicated additional bits needed to build the Bitcoin system that were already well documented elsewhere- For instance, the node discovery algorithm.
On the other hand, understanding _why_ it works is difficult, as it requires understanding the hard math underlying the system. In fact, nobody knows _why_ it works, because, AFAIK, we do not know whether the hash functions used have the necessary properties (in fact, it is worse: we do not even know whether one-way functions exist, let alone that we can point to one)