Having a specified theme allows to have judgement criteria to which the entrants and the jury can relate to. (determined criterias, known from all)
It is realistic to have three top winners over the same theme. It is not realistic to have tons of "winners" in numerous "category". (reminder to everyone that the "Your creation" section is there if you want to post your games)
I mean it would be nice to entrants but honestly, a pain in the ass for those who organize.