Well, there is a reason for this. It’s historical and it’s related to the development of the English language and religious influences in particular. Now, the English word Christ comes from the Greek word Χριστός (khristos) and in old English it was pronounced with a short /I/ or sometimes a long /i:/ so we used to say Kreestos and we still use this old pronunciation in words like Christmas, Christian, Christianity and Christendom. However, in some words we use a more modern pronunciation with an /ai/ sound – and we find this in words like Christ, Christchurch, Jesus Christ,Christ-child, anti-Christ.
I can recommend that you use a good learner dictionary, Tom. They’re always useful for checking pronunciation, and I just want to say thanks for getting in touch and I hope that my answer to your question is useful!