Hey

This is a question that comes up fairly often. I think this article may be of help to you.
I always remember that even in the very oldest MSS there are
slight differences between them such as the Syriac compared to the Arabic. Bible scholars look at all the oldest MSS they can.
It's not just the NIV but ESV, NASB, etc.
The answer is that the translators did not believe these verses should have been in the Bible to begin with. Since the KJV was translated in A.D. 1611, many Biblical manuscripts have been discovered that are older and more accurate than the manuscripts the KJV was based on. When Bible scholars researched through these manuscripts, they discovered some differences. It seems that over the course of 1500 years, some words, phrases, and even sentences were added to the Bible (either intentionally or accidentally). The verses mentioned above are simply not found in the oldest and most reliable manuscripts. So, the newer translations remove these verses or place them in footnotes or in the margin because they do not truly belong in the Bible.
Why are the newer translations of the Bible missing verses?