In the 90s there was a book titled New Age Bible Versions that sent a wave through the Protestant churches, especially Southern Baptist. The book was written by GA Riplinger and went through the various versions of the Protestant Bible (e.g., NASB, NIV, & etc.) and took a near verse by verse comparison of the different versions to the KJV. The author explained in detail why the non-KJV versions were espousing New Age tenets and why the KJV should be the only version a Christian reads. Further controversy erupted with versions like the NLT and The Message.
Some people also only use KJV because of Strong’s Exhaustive Concordance and the ease with which one can find the original Greek or Hebrew used in Scripture.
I think there are a plethora of reasons why one may be KJV only. For me, as I am recently converting to Catholicism, I was in KJV only church for many years, and my parents were (and still are) KJV only. I am so used to the KJV that I find reading other versions difficult. I try and switch out the versions that I read on a yearly basis, but find I come back to KJV - again not because of some hocus pocus belief about KJV, but rather because it’s just what I am used to and can read it easier.