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

Protoss has the most mature human written SC BW bot scene right now. If you look on BASIL ladder much of the top 10 are Protoss.

Protoss vs Protoss only also helps downscope the problem given the time constraints

I can tell you why NOT Zerg. Zerg you have to do too many actions. Select the hatchery, select larvae, choose which unit to build. That extra select larvae step is a problem.

for AIur

toss imba