In short, because it is important to us that anyone that works with Avada on WordPress or visits an Avada powered website has the best possible user experience. IE11 does not provide the website visitor with a modern and reliable experience.
Because web standards and technologies are continually evolving, and so too is the Avada Website Builder, it is important for our team to direct their efforts where it matters most; In this case, modern browser rendering. Browsers like Chrome, Firefox, Safari, Opera and even Microsoft Edge, are evolving and continually being updated and improved to match web standards and this is a good thing. Internet Explorer on the other hand is not evolving. In fact, Microsoft advises against using this browser as the deault browser choice. Futhermore, the Microsoft 365 Apps team is saying goodbye to IE11 by August 17, 2021.
WordPress.org is also in active discussion about when to end support for Internet Explorer 11 (IE11) and will likely be fairly soon. As summarized in this post the current state of Internet Explorer usage among WordPress users and references three metrics that clealy indicate a decline bringing it down to below 1%: