To explain or to be explained
These Google Ngrams show that ... too tough to be eaten ... and ... too tough to eat ... are both used, but that the second variant, with the tough movement, is far the more common.
I'd say that ... too hard to be explained ... is considered stylistically even less optimal. The relevant Google Ngrams seem to confirm my suspicions.
Too difficult to be explained has a very small following.
This Wikipedia article looks at tough movemment.