It's a basic question of fairness. Playtesters have an advantage as they know the rules beforehand. Whether they can capitalise on that advantage is an irrelevance.
It's interesting you display a load of wins and say that playtesters haven't got an advantage because they win with relatively old books, you're making the case that by not playing the new stuff, it won't affect their performance.
Therefore, playtesters not being able to enter a tournament with the new book doesn't cause them a detriment and eliminates the potential for an unfair advantage.
Put another way, if it's no Biggie to play with the new stuff, then why argue that you should be able to, when you could easily not, and protect your reputation.