It's really simple - the early Unix kernel had so few lines of code because the machine had so few bytes of memory (180KB I think), and a bigger kernel wouldn't

It's really simple - the early Unix kernel had so few lines of code because the machine had so few bytes of memory (180KB I think), and a bigger kernel wouldn't have fit. It's actually worse than that, because the PDP-11 address space was 16 bits, so the kernel had to be <64KB. It really only had two types of devices - the disk and the serial ports. No network protocols, and a single very simple file system. Basically no virtual memory - it had segmentation, and most memory management was done by swapping processes to disk. More complexity would have to wait for bigger machines with more memory.