While I may have no answers to this question, I can say that such a question must be strongly qualified. Rather than "Why do men ...?" one must ask "Why do American men ...?" or "Why do Western nations' men ...?" There is no consensus that Indian and Bolivian and Japanese etc men have a similar difficulty with expressing feelings (they may or may not, but there is no body of research that indicates either way).
Additionally, you must ask "Why do men of the 21st century ...?" or "Why did men of the 20th century ...?" Further, unless you wish to prove that men like Dickens, Keats, Byron, Shakespeare, Spenser, Chaucer were aberrations, it is clear that Western men have not always found it difficult to express feelings.
In order to get your answer, you will need to look not to contemporary psychology nor to one single group of contemporary men, but to history and to many groups of contemporary men from Borneo to Norway and from Japan to Cairo to Mexico City.