You are looking at this with a 'pro-rail/anti-air' viewpoint, not unsurprisingly on a Railforum! - there are as many factors against Sleeping car trains, including their poor timekeeping and thus reliability (e.g. due to winds bringing down OHLE and heavy rain flooding the tracks, signal failures, track defects, locomotive failures, faults on other trains blocking the line, engineering works, persons struck by train etc. etc. [ there are massive threads on this forum detailing recent woes]), ease of getting to a Sleeping car station (whether by car or public transport), frequency of services and availability of berths at a reasonable price, the additional charges of getting to the Sleeping Car station and car parking (availability and security), security of stations at night, lengthy journey time, poor standard of room (size, showers, bed) compared to a hotel, rocking of train, noise (movement and other passengers/staff), inconsistency of service on the train (will there be food and drink available?). The list is not comprehensive. For short business trips, the amount of time taken up using a Sleeping car is a significant factor, and it is being a hare that will clinch the business deal before the tortoise gets there.
People are voting with their feet between London & Scotland - far more go by air than by Sleeping car train. I travel by Sleeping Car train as often as possible, because I am a rail enthusiast and enjoy the quirky experience. None of my colleagues are, and would not consider using them.
There will always be some people who are frightened to use planes - but then my Mother was particularly upset by the Harrow & Wealdstone crash in 1952 (knowing several of the injured and dead), and didn't much like going on long distance trains because of it!