PGP might be easier to clone (duplicate private/master keys?) without tipping off the other party like many end-to-end messengers do when you switch devices.

B told Justice Janice Dillon that he sent a message to Kim on a Blackberry that had been cloned from one belonging to the UN associate.

http://vancouversun.com/news/crime/former-gangster-says-dhak...

According to the article the company had been in operations since 2007 or there about, I dont think there was any end2end encrypted mobile messenger back then and they have properly just continued what worked for them.