There is no way to tell exactly why Game Freak did this, it does seem a bit unfair, but you can see the logic why White Forest has wild Pokémon and Black City doesn't.
White Forest also has more characters and tasks that do not appear in Black City.
But, Black City has more trainers and different tasks than White Forest, which evens it out a bit. This makes sense, as Black City is a city and White Forest a forest.
Even so, you will have to ask Game Freak for a solid answer, but they probably thought it was fairly even.