What is the general justification for the hardness of finding preimages for cryptographic hash functions?
The general justification is several years of cryptographic research in trying to "break" hash functions. As far as I know there is no "proof" that finding pre-images of a hash are hard. It is just assumed to be hard based on the history of effort trying to invert it. 
A hash function considered secure today may well turn out to be weak tomorrow. 

This paper (ONE-WAY FUNCTIONS AND CIRCUIT COMPLEXITY) might be of help in understanding the relationship between circuit complexity and one-way functions.