I thought it was a bit surprising that sleeper services were expanded so much so late - up to the 1980s - before dropping sharply for the obvious reason. But that was not just the after-effect of wartime limitations. Looking for a few quotes on the subject, I was surprised to find out that all sleeping cars were first class until the 1920s. And if you look at early timetables you will find a lot of overnight trains, but few of them with sleeping car accommodation.
There were news reports of second class sleepers as a new idea in Italy and France in 1922, and at the same time making them compulsory in Britain was being discussed for inclusion in the Railways Act. Then in Daily News (London) 03 August 1928:
"Third-class sleepers | To be running in September | Coaches now being built | Third-class "sleepers" will be running on the British railways on September 24th. The work of building these coaches is going ahead as rapidly as possible, and it is expected that the initial demand from third-class sleeping facilities is one that will grow rapidly."(Note the quotation marks, capitalisation, etc!)
Clearly the companies were not keen. You might like this
discussion of the impact of amalgamation from 1927 (Hansard), as an interesting comparison with today. In it, Lord Monkswell for the opposition said:
With regard to third-class sleeping carriages, it would be indecent to expect the railway companies to abandon their first line of defence, so we are duly informed that they do not consider there is any demand for sleeping bunks of a simple type. After that we are told that third-class sleeping carriages would add too much to the weight of the trains and would be too expensive. My suggestion is that third-class sleeping carriages with three tiers of bunks of a simple construction exactly like those used in the ambulance trains during the War, should be introduced. The expense of building and maintaining carriages of this kind cannot differ appreciably from the expense of building and maintaining ordinary third-class carriages. They could be made to accommodate about three-quarters of the numbers of passengers which a third-class carriage of the same dimensions will accommodate and as the bunks would have to be reserved beforehand they could be better filled. There is no real difficulty in introducing them, and as for the absence of demand I do not believe a word of it.