Because the 1611 version is very much different. For example Matthew 26.64 in both:
Iesus saith vnto him, Thou hast saide: Neuerthelesse I say vnto you, Hereafter shall yee see the Sonne of man sitting on the right hand of power, and coming in the clouds of heauen. (KJV 1611)
Jesus saith unto him, Thou hast said: nevertheless I say unto you, Hereafter shall ye see the Son of man sitting on the right hand of power, and coming in the clouds of heaven. (KJV "Authorized")While I know it is only a difference in spelling here, if the KJV is the "only version," how dare someone change it. Otherwise, it is just a good, reliable translation. If someone has the date that the KJV became "Authorized," please tell me in the comments section. Because it is not the original KJV we use now.
KJV-Onlyism is heretical and requires the use of a specific version for salvation (as I've been told before, "You can't be saved using the NIV." )
I use the KJV, but also the NASB, NIV (though I think it is weak in many areas), NKJV, ESV, WEBible, NET Bible, NLT (though it is also weak in areas), Young's Literal Translation and the ASV at times. What do non-english speakers use if the KJV is the only version God sanctions?