Many participants would provably do back tracking of what the past winners submission to have basis to create more better artwork to submit. So creating the same style or theme on this contest might not give any best result to anyone so best to create more unique which can possibly can catch the attention of the winner or to the people who will judge unto this contest.
Artist became more creative as the art submissions by users are so good, we can say that this contest really evolve since participants truly extend extraordinary effort to possibly win on the contest.
I think you can clearly see the evolution of these art contests. In the beginning there were not so many participants and nowadays, a lot more people participate and the quality of the artworks is also a lot higher. I don't have any artistic skill but I always look forward to see the winners and the different styles of artwork.
You're right, the number of participants are increasing and their artworks are indeed amazing so it's a bit hard now to win. But of course nothing is impossible and if you love what you're doing, you can execute a nice artwork since it's your passion. It would be better to just have less expectation to win so you won't be too affected just incase your artwork is not chosen. But atleast you did your best and you showed your talent.