Ordinary people could see if a car was taxed or not. Not many would have actually reported an out of date disc but it was the "fear" of being spotted which ensured most people taxed their cars and also, it was a visible reminder to do so. Now, lots of people forget to tax the car so end up getting fined.

Now, you walk down the street and have no idea which car is taxed or not unless you know how online. Not many people will bother.