Why is it sometimes so difficult to determine causation

1) Because of "lurking variables".

2) Because correlation does not necessarily imply causation.

That being said, correlation may suggest causation, but to what extent may not alway be known.

3) This extent may not be known since "unknown" or "missing" pieces of information may be unavailable or need to be researched or evaluated; and and types of biased need to considered in determining validity and reliability.

Some of the causes can be less important than others, but determining their importance is hard since historians can't rely on experiments to determine cause and effect relationships the way scientists can. It's also difficult because events are in the past, and sometimes historians don't have much evidence to work with.