That's mainly because DOS couldn't do anything. If you want to reproduce the capabilities of DOS - single tasking, single threading file and screen IO in text or simple non-windowed graphics - you really wouldn't need to learn much more nowadays.
I'll take a guess and say that that time was well in the past by 1990. I remember the confusion over XMS / EMS and how to write programs that could use extended / expanded memory.
It all depends on how far into the system you want to look. How many users of the FAT file system actually learnt the innards of NTFS? How many programmers specialized on Borland IDEs rather than WatCom? There's not really that much more to learn these days if you want to just write software; understanding the entire system is a matter of jumping down the rabbit hole. If I understand how electron tunneling prevents processor architecture going much beyond the current state of the art, do I have a greater understanding of computers than someone who knows the OpenGL API backwards?
I agree. And in case you have some specific bottleneck you can always go into that. As knowledge grows, it happens in every science.
However, it's another feeling when you have a good grasp of the entire system.
"There was a time when if you read one book by Peter Norton, you literally knew everything there was to know about programming the IBM-PC"