Linux and it's lack of hard real time functionality. ws2812b's don't have a clock signal and instead rely on relatively tight timing of the data to seperate the bits. So you end up having to write a kernel driver to either disable interrupts as you bit bang GPIO, or take over a SPI device and set it up with nasty parameters to get a ws2812b waveform out of the MOSI data line. This chip can simply dedicate one of the MCU cores to the task of babysitting that interface.