I'd say that, and also probably due to the space required for the construction. not to mention the resources.
Since they are still doing primary construction on some of the village buildings, they may be putting the resources to start construction on Snow White on those buildings to get them closer to completion. Then once the primary structure is completed, they move those workers on to the next building and move the next crew onto the village buildings to start the next phase of their construction.
As for space, currently the place where Snow White is going is pretty much right up against the existing Great Wall of Fantasyland. Because of this and how large it will likely be, You don't want to take up the room which trucks and equiptment might be able to use for the construction of the existing two primary buildings under construction.
I also wouldn't be too terribly surprised if they would need to move the great wall closer to Pooh to have the room to construct Snow White.... either due to the physical building location, or just to allow the equiptment room to get to both sides of the construction. Because of that factor, It would make sense to at the very least wait until after the busy summer season to start construction to prevent creating a bottleneck situation. It may even make sense to wait until they've opened up the walkways in front of Mermaid and Beast to allow traffic to go around the snow white construction area.
And then there is always the view. Snow white will be going up on just the other side of the great wall. It will obstruct the view guests may have of the progress of Mermaid and Beast's castle... so Disney may be waiting because they realize the view and excitement of their nearing completion makes the ugliness of construction much easier to many guests to deal with. (as well as being a great marketting tool).