The base for any real-time application usually consists of a wide variety of hardware components with largely different purposes. Peripheral controller chips play an essential role in a hardware design; they are the real base of an application. The system software controlling one of these chips is called "device driver" and is hardware dependent to a high degree.
When implemented for a legacy real-time operating system, the driver software must be ported to new hardware again and again to keep pace with development of new peripheral controller chips and to make them accessible to the application program.
With increasingly shorter technology innovation cycles it's getting more and more expensive to implement and provide hardware dependent driver software.
In the case of a well-structured real-time operating system like EUROS, porting a driver is limited to adapting or re-writing the hardware dependent parts of a driver (usually in form of a Port Driver).