The Bible is commonly used because some Christians consider it a duty to ensure everyone everyone can read the word of God. That means they need to translate the Bible and make it available to people who often don't have money. If you want to learn one of the languages everyone else is learning (this is different in different parts of the world) there is no problem, but outside of that set of 5 there are few resources and many are bad. You can always count on a reasonable good Bible translation no matter what your target language is though. For the vast majority of languages (which are rare languages only used by a small number of people) the Bible is the only option.
Also, for Christians, the exact wording of the Bible is critically important. Well-used translations for any significant language will be scrupulous. The problem is the language used will be archaic, because changing the wording is a big deal.
Compare that to https://wycliffenz.org/about/our-work/bible-translation-stat... which claims 683 full translations, 1534 new testament (complete enough for language learning), and 3350 with at least something translated (maybe not enough to learn the language)
