As I've heard it, it's motivated by security rather than usability: If two people use Signal to communicate with each other, then they should use the Signal pro

As I've heard it, it's motivated by security rather than usability: If two people use Signal to communicate with each other, then they should use the Signal protocol rather than SMS, without having to go through any extra steps. There are different ways to achieve that, all of which have drawbacks. They chose one.