Because it's literally a physical=desktop keyboard, just smaller in size while almost all current mobile interfaces are touch based?(also, the question wasn't about uniqueness, but productivity levels of a desktop productivity app, think about code editors with extensions, keyboard and mouse gesture customization.
For most of their existence, Blackberry had no touch interface. One appeared in later versions as they tried to compete with Android and iPhone. One example of a "mobile keyboard" shortcut was long pressing a physical key to launch a specific function.
It might be hard to understand now, but Blackberry power users could be much more productive with email/texting than any phone that exists today. But they were special purpose 2-way radio (initially, pager) devices that lacked the flexibility of modern apps with full internet data access.
What did they have in their touch interfaces?