Why? Because some one must be the scape goat. Remember, its the 21st century! someone must be to blame, and certainly not ourselves! Ergo, the powers that be must pick (seemingly at random, although to be fair I have not seen a train driven through two huge buildings killing 3000 innocent lives yet) an industry to implement over stringent regulations upon. You must remember a few points, there are boot loads of train stations world wide, not so many airports, Airport security on all train stations would be 'infeasible' at the least. Plus! In doing so, they create a new industry out of an industry that is in a huge state of growth already. If there is money to be made, you bet someone will try to do it. Politicians are not going to complain, as it supplies jobs, always a sure winner for votes.
I do have a question though, for those nautically minded and experienced. Do cruise ships go through the same type of thing? And what's security like for those HUGE butane tankers etc?