It's partly because it's easier to judge. Taste is subjective and the viewer can't participate nor cares. But presentation - or how the cake looks? That the viewer can participate in and does care about. Even the Great British Bake Off has swung more and more towards presentation in later years.

I mean - the audience can see if the cake really looks like a hamburger. Plus there's the added bonus of not wanting to eat it. For a reality show competition it's more fun.