I can think about few reasons ...
- First of all you need to prepare the world for all classes, and its much easier to retrospectively edit smaller world, than larger ...
Just imagine you are suppose to go through 15,000 conversation lines just to check if they all are valid for Druids ... a lot work to be sure, but still kinda less than if you are suppose to go through 30,000 conversation lines. etc.
- Second, usualy in developing process you want to have everything done first, before you move futher.
That means, concidering previous point ... you first want to have the world prepared for everyone, and then, when you are satisfied with how everything is working out ... then you can move to another scene. Making it oterwise around cause problems.
- Third, we didnt get "just druid" even if we would count only things that are conected just to druid directly and will ignore those spells (30 was anounced as far as i know) that will be only for others ...
You still get another one more or less unique experience with many more interactions, options, etc. ... and as i said, i only count druid things ... meaning i ignore things like Speak with dead update, or lights, and who knows what else is there.
- Four, i bet that implement new class is a much faster job, than implement rest of the act ... especialy since we dont even know how much of Act1 we allready explored. What we curently have could be aswell 95% of whole Act1 as 50%.
- Five, lets be honest with each other ... druid will simply keep us bussy, and its kinda carrot on a stick, that we dont go futher in our story.
- And last, but certainly not least ... im not quite sure we even should get "rest of Act1" ... nor even "soon" ...
We were told that we will have limited space, levels, and kinda everything else ... but as far as i know, nobody ever promised that we will get Githyanki camp, or Moonrise towers in EA. O_o