It's my understanding that the WS2813 is very sensitive to the timing of the digital signals from the controller. As Linux is a non-deterministic O/S, coupled with the multi-processor architecture of the raspberry pi, you simply cannot control the digital outputs precisely enough to get the timing required. Plus, the output current on the pi is constrained as well. While this may not be an issue in regards to the WS2813, it may be for other projects.,
Although the pure processing power of the Arduino is much less than that of the Pi, the Arduino is a single-processor micro-controller, where you CAN control the timing of the output signals.
If you need to use the raspberry pi for other reasons (for display, logging, and user interface, for instance,) you can use it in a supervisory role, serial connected to the Arduino which handles the direct I/O.