Mainly because it cannot be trusted enough for this kind of application.
It is not Open Hardware. The processor used is very closed, even the data-sheet is not accessible to the public. (It's very problematic for a device like Own-Mailbox very sensitive regarding politic.)
The Raspberry Pi is provided with non-free software and the hardware needs non-free driver to work. (Also problematic.)
Raspberry Pi is bigger and less energy efficient.
On top of that, we don't use it for prices reasons, people have already tried to sell all-in-the-box devices based on Raspberry Pi and the end up being expensive.