To stop the debate kicking off and having catfights all over the place, there are a number of reasons you have to consider.
Remember 90% of the train-sim market are not on any railway forums and are simply hobbyists who go out and buy addons from the shops. Fact. Great Eastern & London South East are one of the most well known addons available in the world and hence you go to the Making Tracks website and see "ooh, a 66, let's buy it". It's called impulse purchase. Remember, you are actually just paying £4.99, because £5 is the Railwaves sound pack. On the otherhand, as DTD is freeware, not everyone will have heard of it in the UK...