Maybe because Protosses have been doing bad lately and need some help improving :) (won only 3 out of 22 ASL championships in the past 10 years)

On a more serious note: all three races play quite differently so it makes sense to focus on just one. And protoss is mechanically the easiest and most straightforward race: fewer but stronger units, a single worker can be used to build multiple buildings at once, versatile basic static defence