For most of the time the Internet has existed, SEO practices have largely revolved around search engines indexing plain text version of the website content and not content that has been dynamically generated using programming platforms like JavaScript or its hugely popular library, jQuery. According to Forbes.com, search engine crawler bots are not up to the mark when it comes to understanding JavaScript, and consequently, website content indexing may not be performed completely, and the rankings pulled down.
Source: Wikipedia
Translated from Dutch with Google Translate