This is the most common reason. Many great mods (trucks, maps, and AI traffic packs) created by the community were hardcoded for specific versions. When SCS Software updates the game to a new version (like 1.36, 1.37, etc.), it often breaks these mods. If your favorite truck mod was never updated past 2019, Version 1.35 is the only place it will function correctly.
If you are looking to download and install ETS2 Version 1.35—whether for mod compatibility, nostalgic performance, or system limitations—this guide covers everything you need: new features, step-by-step download instructions, and important compatibility notes. Download Euro Truck Simulator 2 Version 1.35
You cannot download version 1.35 from the Steam store page directly (only the latest version is shown). However, Valve allows you to roll back to any previous version via the "Betas" tab. Here is how to do it: A Long Story: Downloading Euro Truck Simulator 2 Version 1
In the "Beta Participation" dropdown menu, scroll down to find . If your favorite truck mod was never updated