First of all, if you are begging him to be honest with you then it's time to move on. If you don't have honesty and trust in a relationship it's over before it gets started. Think enough of yourself to get on with your life. You'll be glad you did after awhile.
As to the "why", there are many varied reasons. A dishonest person will sabotage a relationship so that he/she can tell themselves that the other person broke it off and they are not to blame for the breakup. Also, the male ego can be very fragile and will cause a man to deny even to himself that he shares in the blame.
The "why" isn't important, but your self-worth is, so make a decision to make your life better.
Good luck to you.