I have no idea, but they did. There was tons of discussion on it at the time the device was first released and you can also see the FCC filings.
There was some supposition at the time that the companies agreed intentionally that they were going to make devices that would not work with other ADT panels in order to distinguish them and prevent cannibalization of the professionally installed products. But I never saw anything official on that. All I know is that they won’t work with anything else.
But we’re getting pretty far off topic for this thread, the discussion should go in any of the existing ADT threads.