Licensing. They are trying.
FIFA has tons and tons of generic faces. More leagues, more teams, more generic faces. PES has added over 200 faces since launch. So you can expect greater improvement all the way until 2016 launches and so on.
The PES staff is no where near as large as the FIFA staff, thats why I think things are taking a lil bit long in comparison to FIFA. but give it sometime and hopefully 2016 they will really bring it home.