Simply put. Many of these Modern Translations, do not come from the same manuscripts that the King James Bible was translated from.
The King James Bible originates from what is commonly known as the 'Received Text' or the manuscripts that had originated from the original writers and been copied and re-copied and duplicated for the wider reading.
Many of the Modern Translations come from what is known as the 'Critical Text' basically from two major source manuscripts, one of which was recovered from a bin outside a Monastry. These were called the Siniaticus and the Vaticanus. (NT Greek) and were riddled with alterations and critical changes, hence the title 'Critical Text'. It was certainly older, than the Textus Receptus or Recieved Text, but that was only because the texts had not been copied, my guess is it was because they were a mess. Two Cambridge professors Westcott and Hort, had a go at working on the Critical Text and surrendered a New Greek Translations which had an English translation, along with some work on a book called 'The Shepard of Hermes' The two professors were basically very flamboyant fellas and like to experiment with different ideas to revolutionize the text. Ofcourse, many of the texts from the manuscripts had been already tampered with and with the aid of numerous reworkings, and Bible was produced called the Revised Edition 1881. The texts that were missing revealed a definite desire to obsure certain prime doctrines, like the Trinity and certain aspects of the message of salvation. For a more detailed study, have a look at the seven year work by Professor Riplinger called, 'In Awe of Thy Word' She noticed these blatant errors whilst attempting to counsel some of her students.
I have spent some time examining the History of the Roman Catholic Church, and the path of blood and hatred that was directed at those who possessed the Received Text, or Textus Receptus, or any Bible other than their Bibles, and certainly there is a sameness about the Rheims Duo, the Jerusalem Bible and the Modern Bible that have flowed out of Westcott and Horts original work.
Though the King James is really difficult to get around the English, the sourses from which it came do much to help one into a good understanding, though weak grammar will help no one to understand what is written in the bible, for it is definitely litreture, from God, and dare I say it without seeming disrespectful. Reading a book written by God, has got to be an experience of literary amazement. One read of the Song of Solomon, certainly taught me that God really knows plenty about sex, he invented it after all. Peace.
The link to Proffessor Gail Riplinger, should be easily enough to find on the internet.