Canon extenders are limited by both physical design and performance concerns.

Physically, Canon teleconverters have a front optical element that protrudes forward of the mount. They only fit lenses whose rear elements do not extend too far back into the mount area. Many incompatible lenses, including EF-S designs, use that rear space, so the extender would interfere.

Optically and operationally, extenders also reduce the amount of light reaching the sensor and autofocus system. That can hurt AF reliability and overall image quality, especially on lenses not designed to work with a teleconverter. So even if a combination can be mounted, it may not perform to normal expectations.

Third-party extenders may use a different front-element design and can sometimes fit lenses Canon extenders will not. But fit does not guarantee good results; image quality and autofocus support may still be compromised.

So the restriction is not arbitrary: it’s mainly about rear-element clearance, plus maintaining usable optical and autofocus performance.