I don't really think bitcoin is hard to understand, but I think how it works, the blockchain technology around it, the mining, and sort of technicality is the hardest to understand. But understanding bitcoin is so easy, you just have to know how you can use it, and to understand that it's a currency. I agree with you about having textbooks for bitcoin, and I think there are already, thousands are here in the internet too, you just have to search and take a lot of time reading about it. Forum just like this is of course helpful, that's the point we are here right.
I think aside from having textbook, it is better to learn about bitcoin in videos, its better to understand in that way. Documentaries about bitcoin create simple introduction to bitcoin. There are a lot in youtube too. I don't know but I think videos are a lot more helpful cause it helps you visualize technicalities, instead of struggling to read and be drown inside a thick textbook.
Those for whom it is difficult to understand Bitcoin, didn't read enough literature, didnt' try to really delve into the topic and figure it out, and simply didn't deal with it. There are actually a lot of books about this, it is also interesting and important to know the history. In many countries, advertising for bitcoin is prohibited, and people cannot pay with BTK, and while bitcoin owners have no freedom, many people will have no idea about bitcoin at all and the word cryptocurrency will say little for them. But question: is it a real problem for now?