I don't work on or speak for the Opa team, but I assume they adopted JS-like syntax for the same reason Dart did -- familiarity is important when it comes to language adoption.
Regardless of whether or not you think this is a good thing or whether programmers worry too much about superficial syntax issues, this is a practical issue you have to worry about if doing PL design for the real world.
Changing syntax for marketing reasons seems bad from my point of view, I like pythonish syntaxes, like in haml, CoffeScript etc. So changing to JS was a back step.
