Because the Love of God is not in them.
For God so loved the world, that he gave his only begotten son, and whosoever believes in Him shall not perish, but have everlasting life. John 3:16
Just imagine, if they don't acknowledge nor worship the Son of God, and when one of their members leave their organization, they're no longer accepted, as if that person never existed in their lives. This is pure evil and hypocrisy. God commands us to love him with all of our hearts, mind and soul, and love our neighbor as we love ourselves. So, if the JW's can easily receive you as one of their, but once you accept the Lord Jesus Christ, and find their hidden agenda, they will throw you out the door, before you can blink. Like I said, they DO NOT HAVE THE LOVE OF GOD, for God loves everybody no matter who you are. God doesn't discriminate, but religious organizations can say who stays or who goes. And, this is not of God.