psychological subjection, if theymos makes a pizza, many will be tempted to vote for theymos' pizza even if it's not the most beautiful pizza
to have voters who vote objectively, in my opinion, we need to publish the pizzas anonymously so people can really vote for the one they like
Publishing the pizzas anonymously would require more effort in coming up with the best way to do it, and in my opinion it is going to reduce the fun in the contest. While i don't think Theymos would participate, but if he does, and he gets the most votes, then he will be one of the winners, there is no need to complicate things.
And knowing him, he might just donate the prize money to a trustworthy charity organization. He might also use it to help the forum. $2500 in @theymos hands isn't exactly a bad outcome if he were to win.
What if he likes pizza lol?
You made a very good point @Z-Tight, about overcomplicating things, ya know?
Have they ever been anonymous in the past?
Too much work to make bitcointalk pizza contest submissions anonymous!

i said theymos like example, because theymos is admin and he do this stuff, partecipate and donate
because if I name someone else then they might be offended and a drama worthy of this note would arise