There's no criteria. A long long time ago, models only consisted out of a few bricks. (It was quite complex to build with Mecabricks a few years ago) And because of that, qualitative models were selected to hide the models with just a few bricks. But times have changed and making LEGO models with Mecabricks is much easier. And thus, now a model get 'randomly' selected. In fact, originality and good-looking play an important role in the selection, but it's not a must.

Thanks for the info Sim!