With Platform 9 3/4 in King's Cross Station, it means all the magic people have to take the effort to dress as muggles (according to the International Statute of Secrecy) and somehow make it to King's Cross Station without revealing themselves (meaning they can't just use apperition and appear wherever they want in King's Cross).
Considering the number of people who will need to do this every year, at least twice each year, and that they will be interacting, at least minimally, with muggles, this creates an unnecessary risk of tipping off muggles something is going on. (Not to mention all the people walking through a wall into Platform 9 3/4!)
They have to have a track that is for their use only (or at least parts of it, since they don't want muggle trains pulling up in Hogsmeade or near Hogwarts). Why didn't they either create their own station in London or create a place for apparition so there would be no interaction with muggles for all the Hogwarts students and their families?