I don't know what data they are trying to extract out of this alpha, if it is just matchmaking and performance stuff then I can see how adding all the characters might not be too high on their priorities list. Still, if they are in a playable state (which I can only imagine is the case) then I can't think of a good reason to exclude them.
The more people who play the other, what, 6 heroes, the more they can fine tune the balance in the coming betas and tests. The more they get to fine tune it, the better balanced the game will be when it officially releases and that is to everyones advantage. With luck, there will be many people new to the genre and nothing will turn those people off faster than broken characters or otherwise unbalanced gameplay.
I am not too good at this game and I am new to the genre in general, so I can't make an in-depth analysis on the state of the balance. However, it feels like it is fair, and I have all the tools I need to be competitive with practice. Is this the case for the other 6 heroes? Who knows, we need to play with them to see.
In any case an explanation as to why they are not included would be welcome. I am sure there is a very good reason.