First of all, Bible translators almost never do their own research to piece together an accurate and complete text of the Bible. We have thousands of fragments or whole books of the Bible that are from varying times of history and from different places around the world. Some are even ancient translations into other languages of their day. The common denominator of all of them is that none of them are the original pieces. They are hand-made copies of copies of copies. No matter how careful or well-meaning the copyists were, they were bound to make mistakes. These mistakes were transferred to the next copies as the next copyist had no way to know what the original said. This pattern formed geographical families of fragments all containing the same mistakes. Today, by comparing these families, we can pick the mistakes because what appears in one family does not appear in all the others. So today we have an almost 100% clean text. The job of a scholar is to create a critical text (an accurate text) out of all these fragments that are now scattered around the world in museums and private collections. As time goes by, newer fragments are discovered and newer critical texts are made that are even more accurate.
Next comes the job of a translator. His job is not to re-do what the scholars above did. He uses these critical texts that contain some minor ambiguities and decides what to include in his translation. Sometimes, they include their choice in the text and the variation in a footnote.
Concerning your question, the reason as found in the Preface is:
"NU-Text These variations from the traditional text generally represent the Alexandrian or Egyptian type of text described previously in "The New Testament Text." They are found in the Critical Text published in the twenty-seventh edition of the Nestle-Aland Greek New Testament (N) and in the United Bible Societies’ fourth edition (U), hence the acronym, "NU-Text." "
The reason why the scholars decided to omit "adulterers" is probably because it is not found in some manuscripts. You can download the critical text "the twenty-seventh edition of the Nestle-Aland Greek New Testament" from the Internet and see how they explain the omission. All I can tell you is that the New World Translation made by Jehovah's Witnesses also omits the word. And they are very careful in their translation checking the minutest detail. You can trust them. In my opinion, your Bible is correct in omitting the word "adulterers". Probably James wrote only "adulteresses" and some overzealous copyist was enticed to add "adulterers" for more clarification. It's the type of error we see hundreds of times in Bible manuscripts. They are called "interpolations". Also, this critical text is, as far as I know, the most recent one (Stuttgart, 1996).