Why do men do this ?

This is not exclusive to men. In this modern dating world where men are expected to approach, initiate and carry the conversation while women do none of this and very passively decide which men they want to talk to and which they want to ignore - the problem of women not replying is much more prevalent than men who do not reply. Only in very rare exceptional circumstances do men not reply, and yours is one of those exceptions. So don't make out that men are the problem when you just can't be bothered to learn and understand how men think and operate.

Let me make it simple for you. The only reasons a man does not reply are:

A) He is not interested in you.
B) He is interested in you, but he's getting dead replies from you and the conversation is going nowhere.
C) He is talking to other girls who are giving him more enthusiastic responses and hence he gives them more time than he gives to you. However, most guys do not receive attention from multiple girls in the same way that most girls receive attention from multiple guys, so this is a rarity.

The question you should be asking yourself is how do you really feel about him not replying any more? Are you upset about it because you're interested in him and you want him to talk to you? Then you need to be interesting, engage in a conversation and give him a good reason to talk to you. If you're not interested in him then why complain that he isn't messaging you? I'm sure most girls would be relieved when a guy they are not interested in stops messaging - it's one less person filling up your inbox.

First off he never not reply’s to me. He always replies to me ! My question was why did he stop initiating contact ?

Are contact ended then he begged to talk to me again. Then are contact ended again. Why do men beg for you back just to let the contact die?