Any reason for non-continuous mountain road barrier?
While this deals with medians and not strictly side barriers, the animal answer is strongly supported by this study:
https://www.researchgate.net/publication/256425278_Potential_Impacts_of_Highway_Median_Barriers_on_Wildlife_State_of_the_Practice_and_Gap_Analysis
A reasonable response would be that there is a drop into a stream that would prevent animals from crossing.  And I agree, and suspect that simple legality is part of the answer.  A law that requires an animal passing gap every X meters may simply have no exceptions for areas where terrain would be impassable for the animals.