Actually I don't know why the politicians haven't done this.
Trains do not make them money, but road transport is big bucks to the pollies, (fuel alone etc) why do you think the Govt got rid of railways. There money bin is never been so full. Who lobbies the Govt if Road Transport has a hiccup, or there is slight threat to it.
Remember the original "Railways of Australia" that was to be when it was first though off, was to be one gauge, one organization, the Road and Rail could have
run hand in hand. An easy workable situation. "That train now dun gone
Opps, as usual the damage is done and the answer is far to easy to fix, to simple, but govts do not like simple answers. Those who think they are important, like complicated answers, makes them feel important and solves nothing. But to reverse it now would cost huge bucks to correct, so its the cat continues chasing its tail now.
Plus the Govt invented a new word "Competition", Hmmm just look at our Banks, Electricity, Water, etc etc etc, all in the name of competition, but what Competition,,,,, where......... only to see who can compete for the highest price and more profits for those who invest in the stock markets, not the users. There is No control, No regulations, just open book to regulate yourself and rip the system off at any cost. Sorry just sore factual point that we (the ordinary man in the street) all know this and cannot do sweet nothing about, but talk about WHAT... SHOULD.... BE.. and isn't.
The truckies can't represent too big a slab of the voters, and it would be big money...
It has nothing to do with Voters. IF you had a Govt who went to the polls with lets put semi's on Trains with trucks off the road as its main agenda and they were voted into power, nothing would change. Oh there would be several million dollar programs to look into it, but as time went on nothing would really change, and in 10 years you and I will still be saying, "this is what it should have been" and we come back to your first question, "Actually I don't know why the politicians haven't done this
All this said, I'm not against Road Transport, as everything has its overall part to play that is best for all, and the environment also. We just got to grin and bear it, and meantime gives us something to do typing, "Why Is It So"