Unclear why it's "technically impossible" but from this Washington Post article [1] it sounds more like "prevented by business logic," at least according to their source:

> A person familiar with NSO operations who spoke on the condition of anonymity to discuss internal company matters said Sunday that +1 phones are safe from Pegasus no matter where they are in the world. The system is programmed to block efforts to hack them, the person said.

> The person also said Pegasus can determine where a phone is geographically and block any efforts to hack a foreign-registered phone while it is inside the United States.

> But there is no way to determine the nationality of the user of a phone registered to a foreign system.

  [1]: https://www.washingtonpost.com/national-security/2021/07/19/us-phone-numbers-nso/