This is likely to be because of the increasing security needed for air travel. In the UK most journeys are sufficiently short that any time the plane gains over the train by being quicker is immediately lost to long check in times. Additionally trains are generally a bit comfier and business travellers find it easier to work on trains using laptop charging points etc. which are provided by the like of Virgin.
The main downside of the trains is that when they go wrong you tend to end up stuck for quite a while somewhere in the middle of your journey, which is less likely to happen on a plane...