1) Safety. In the 20th century, safety was mostly passive. Today safety is active. For example, vehicles now use ADAS (Advanced Driver Assistance Systems). If an independent shop replaces a windshield but doesn't have the proprietary software to recalibrate the cameras behind the glass, the car's Automatic Emergency Braking (AEB) might see a phantom obstacle at 70 mph and slam on the brakes. Manufacturers argue (correctly in my view) that they cannot be held liable for a car that decides to crash because a third party misaligned its digital vision.
2) Cybersecurity. There was no backdoor to a 1950s tractor because it wasn't connected to anything. Most modern vehicles are telemetric, where they are connected to the internet via cellular 5G. Manufacturers argue that if they provide open diagnostic ports to everyone, they are effectively creating a standardized digital doorway that a bad actor could use to remotely disable a fleet of tractors across the entire farming sector. In their view, right to repair creates a national security vulnerability that didn't exist when machines were dumb.
3) Environmental. If John Deere is forced to provide software tools to access the engine, a farmer might buy a "delete kit" online that uses software to bypass the Diesel Particulate Filter (DPF). This allows the tractor to run more powerfully and skip expensive emissions fluid (DEF). Deere is understandably legally terrified that the EPA will fine them as the manufacturer for facilitating emissions tampering. They are using this regulatory shield to argue that the only way to save the planet is to keep the software under lock and key.
4) The 1975 Magnuson-Moss Warranty Act. This gets interesting. Back in the day manufacturers tried to say, "If you don't use our oil/parts, your warranty is void." The Act legally protected the owner's right to use aftermarket parts. That law was written for physical parts. It doesn't say anything about digital handshakes. Today, a manufacturer can't void your warranty for using a 3rd party air filter, but they can make the engine refuse to start unless a certified digital code is entered by a dealer-authorized laptop.