Why lock characters during an open beta that will wipe?

I'd like to start by saying I'm really enjoying this game and I really hope it does well.

One thing I don't understand about the beta is why would you lock characters away from people through challenges? It is obvious this is already a kind of balance test, so would you not want to have every character be able to be tested from the get go? I don't really see the logic, especially since it has been said progress would be wiped.

As a side note, I don't think the current system to unlock characters is best. I do like the unique take as an alternative to just buying them though. An example would be to unlock Dragon: play 5 games as LLC. The problem I see with this approach is that most of the characters in a "pantheon" aren't the same playstyle, so for example I'm being forced to play either a sniper or an assasin to unlock a melee brawler. I think there are better alternatives to the character unlock requirements since right now it's forcing you to play characters and waste time when you'd maybe rather just play a character or role you enjoy.