If you’ve ever wondered about this manuscript question, consider this helpful note from the ESV Study Bible:
“Some ancient manuscripts of Mark’s Gospel contain these verses and others do not, which presents a puzzle for scholars who specialize in the history of such manuscripts. This longer ending is missing from various old and reliable Greek manuscripts (esp. Sinaiticus and Vaticanus), as well as numerous early Latin, Syriac, Armenian, and Georgian manuscripts. Early church fathers (e.g., Origen and Clement of Alexandria) did not appear to know of these verses. Eusebius and Jerome state that this section is missing in most manuscripts available at their time. And some manuscripts that contain vv. 9–20 indicate that older manuscripts lack the section. On the other hand, some early and many later manuscripts (such as the manuscripts known as A, C, and D) contain vv. 9–20, and many church fathers (such as Irenaeus) evidently knew of these verses. As for the verses themselves, they contain various Greek words and expressions uncommon to Mark, and there are stylistic differences as well. Many think this shows vv. 9–20 to be a later addition. In summary, vv. 9–20 should be read with caution. As in many translations, the editors of the ESV have placed the section within brackets, showing their doubts as to whether it was originally part of what Mark wrote, but also recognizing its long history of acceptance by many in the church. The content of vv. 9–20 is best explained by reference to other passages in the Gospels and the rest of the NT. (Most of its content is found elsewhere, and no point of doctrine is affected by the absence or presence of vv. 9–20.) With particular reference to v. 18, there is no command to pick up serpents or to drink deadly poison; there is merely a promise of protection as found in other parts of the NT (see Acts 28:3–4; James 5:13–16).”
The observations listed here are hardly conclusive, but they provide some insight into the caution issued by many translations.