Because the 2 images use a different texture so they cannot be batched.
Assume you have 2 images A and B each using a different texture added as you did in your example.
When starling traverses your display list in the first instance it would see the following pattern
A,A,A,A.... x 100
Because each item can be batched with the one before it they will all end up in one draw call.
In the second example the following will happen.
A,B,A,B..... x 100
Because A cannot be batched with B (different texture) then it will result in a draw call for each image.
So either use a texture atlas, i.e. both images in the same texture.
Or change the structure so it get rendered in the order
A,A,A,A.....B,B,B,B... x100 with all the As grouped together and the same with the Bs.
You could achieve that by having to sprites one on top of the other, one to hold all the As and one for the Bs, you will have to manually sort out positioning of each pair which is more complex but should still work but a shared texture is deffo easier.
Not sure if that helps, but draw calls can only be batched if they don't change state, ie different texture or material etc...