This is because conventional methods are based on the similarity between a translated sentence and its reference translation, and they give the translated sentence a high score when the two sentences are globally similar to each other in terms of lexical overlap.
in the case of the above example, the most important thing to maintain a high translation quality is to correctly translate "for" into the target language, and it would be difficult to detect the importance just by comparing an MT result and its reference translations even if the number of reference translations is increased.