I went to Stuttgart from London and back in July which takes about 6 hours by train vs the 90 mins of flying and I chose getting the train for various reasons. Some reasons why people prefer getting the train as follows.
1. While flying is quicker, you spend a lot more time going through security at airports and boarding procedures on planes is takes a longer time than just simply boarding a train and that's it.
2. At the other end you still have to go through border controls and get another local train or bus into the city, which eats up more time. With the train you will arrive smack in the city centre and once you're off the train, that's it (for certain countries anyway).
3. With most airlines, the flight itself may be cheap but you may have to take in considerations such as baggage which adds on to the cost, whilst with trains it's free.
4. You have the freedom to move about on trains. I think in some cases if the train is standing at the station for a while you can have a quick smoke on the platform where it's still legal.
5. It's less physically daunting. By that I mean people can get sick on planes, but very rarely you hear about it on trains.
6. On trains you can pass some beautiful scenery and people take the train purely for this reason alone. On planes you're lucky if it's not even cloudy.
7. It's far more friendly for the environment compare to flying and considering the scares these days, it's understandable why people go for the rails instead.
8. People just like the experience! There's something really cool about breezing through countries by trains getting to see a lot of new places up front even by just passing through