Well, most of the stations are built as and primarily function as trade outposts, to facilitate trade between space-bound starships and the planet below. Putting a trade installation way out in the middle of nowhere far from any source of actual trade goods makes little sense. Likewise, tourist stations should orbit where there's a nice view of the thing people are coming to visit - the planet below.
Some stations are (ostensibly) military outposts. These too are pretty useless at the Lagrange points, since they're too remote to come to the aid of the planet if the planet is attacked (or to bring the big guns to bear on the planet below if the planet decides to revolt). So those wouldn't be placed at the Lagrange points either.
Your typical sci-fi Lagrange-point station is a city-in-space, a destination in and of itself, which was built mainly to house the surplus population from an overcrowded Earth. But in the ED universe, we have no need for such places. "Natural" Earth-likes are not all that uncommon, and terraforming is relatively cheap, quick and easy, so most people live down on the surface of these planets, or in the vast underground cities on the Industrial and High-Tech planets, rather than in space.
The only stations that would make sense being placed out there are the research facilities - scientists looking for somewhere quiet, far from nasty gravity wells and most of the interrupting travellers and casual visitors, where they can conduct their research in peace.
