The idea was that blitz mode is already stressful enough, so it's better not to make it even more stressful by adding the risk of getting an unfavorable Castle. Many players have a strong emotional connection to Heroes, so to appreciate a new game mode, they ideally need to either win or at least play well in the first or second game. A Castle that is enjoyable to play and that players are familiar with contributes to this. The second reason is that the balance between Castles is interesting, and allowing free choice makes it possible to quickly gather player opinions and test them in practice. Of course, using the "trade-cherk-olrandom" mechanic is highly encouraged, but forcing it seems undesirable to me.