Most Helpful Guy
why do "guys" not have the guts. You are generalizing when in reality you are talking about your specific ex and possibly a bunch of others.
But to your point more specifically when people don't want to give you a specific reason it is because they are too afraid to be honest and deal with the reaction. It is a bit of a cowards way out (particularly if the dumpee asks for specific reasons), a way to avoid confrontation or perhpaps a way to not hurt a person's feelings, but regardless it is a ploy to avoid being completely honest.
To not even address the person directly, like your guy did shows that he is immature and probably a bit of a coward