It always depends on your goal. If you want a quick prototype and are good with javascript, get a board that you can program easily with javascript. But maybe you want to learn how things really work on a low level. Or maybe you prefer to run on batteries for a long, long time. Or perhaps you need the system to have a constant low latency.

It's like, why use other programming languages when $lang is so good. But there's always a possible reason. In the end everything is a compromise.