There are 30,000 Christian sects
And Jehovah's Witnesses are unique in their outright rejection of the New Covenant, the one that offers forgiveness of sins.
In Him we have redemption through His blood, the forgiveness of sins - Eph. 1 : 7
Then He took the cup, and gave thanks, and gave it to them, saying, “Drink from it, all of you. 28 For this is My blood of the new covenant, which is shed for many for the remission of sins. - Matthew 26: 27&28
Jehovah's Witnesses reject the Blood of the Covenant for themselves personally. This is fundamental to what being a Christian is. This is the primary reason why they are not Christian. There IS NOT ONE NEW TESTAMENT SCRIPTURE that describes how a person can get their sins forgiven outside of the New Covenant.