Why are some verses missing in the NIV?

I'm wondering why some passages in the NIV are missing when compared to the KJV, like: - Matthew 17:21 - Matthew 18:11 - Mark 9:44 - Mark 11:26 - Mark 15:28 - Luke 17:36 …and more. Also there are some passages where the NIV deletes some phrases as in Matthew 5:44: > NIV: But I tell you, love your enemies and pray for those who persecute you > > KJV: But I say unto you, Love your enemies, bless them that curse you, do > good to them that hate you, and pray for them which despitefully use > you, and persecute you I don't understand the need to change the Bible from what it was for so many years; I personally find the old versions really rich in culture and those express much more than the new versions do. Also in the old versions, the points and commas were really important to not change the original meaning. And the Bible teaches that God is the one that gives us revelation through the Holy Spirit to understand the scriptures, so there's no need to change the Bible to make it 'more understandable'