The reason is simple: Along with it's intuitive interface comes a complete set of tutorials and resources for learning how to work the program. For example, the help included with the application.
Rhino is a tool that allows you to work with different types of virtual elements: solids, surfaces and NURBS curves (a mathematical model that obtains very realistic results).
On the author page you can refer to creations made by Rhino users, some are so spectacular they will make your jaw drop.
This is an application intended both for industrial design such as vehicles, ships, architecture or aeronautics.
This trial version is completely function, it even allows you to add plug-ins. The limitation is in the number of times you can use the save option, a total of 25 times.