Because you don't have many people and/or residents.
To get more Pokemon to appear in your White Forest, you need to get more people to live in it. The more people you have, the more Pokemon appear, as there is one Pokemon that will appear for every resident in your forest.
To invite more people to your White Forest, connect to someone's Pokemon Black with the Entralink and invite their residents over from Black City. You can have a maximum of 10 residents, so that means ten Pokemon at once. So if you want every Pokemon from White Forest, you are going to have to catch all the Pokemon, wait for the residents to move out, and get another group of different people in your forest to get different Pokemon.
Residents will leave if you don't enter the area or talk to them enough, so make sure you visit regularly to keep your residents!
This will list each resident and what Pokemon they will bring into your forest.