To figure out usually means to find out…..although on Dictionary.com, it's defined as "hard to understand or solve" with none of its definition close to "to find out". What puzzles me more is the example it gives for the meaning "hard to understand or solve is "we couldn't figure out where all the money had gone." (Obviously, neither of these meanings apply here; solve is close)
So if "inscrutable" does mean hard to figure out. It will be hard to understand, which conform with all of its other meanings. It would be weird to see it mean "hard to find out."
Best Answer
I would say that figure out and find out do not actually mean the exact same thing. It's not easy to explain the difference, but 'find out' usually means that information was given to you (for instance, if your friend calls you to tell you a piece of information, you could say that you 'found it out'), whereas 'figure out' means that you arrived at information without outside help (so you figure out the answer to a math problem).
Inscrutable, then, works better with 'figure out' because it means that you could investigate something forever (literally or as a colloquial exaggeration) and never come to the answer. So you might call a math problem 'inscrutable' if you personally can't determine the answer -- if someone else gives you the answer, that doesn't make the problem less inscrutable to you.