New users should have choice between clients right from the start. I can imagine that many people can be discouraged by slowness of the default client. This oversight don't do anything good to Fizzer.
You're right it's a big problem.
However I don't think giving users a choice right off the bat is a good solution either. If you ask a first-time player if they want Flash or Unity they'd just be confused, as they don't have the context needed to make this decision.
It defaults to Unity since Unity, while it's slow, it at least works on all major browsers. Flash, on the other hand, is blocked by all major browsers by default now, and a first-time player would have to go through a bunch of complicated steps to unblock it. All non-technical people will fail and be turned away by this. So it defaults to Unity.
I'm working on a better solution to this.