Why are compartments in western European day trains falling out of fashion? [closed]

In the past most railways in Europe ran carriages that were built to a common design. These standard UIC designs are still quite common in Eastern Europe, but are disappearing everywhere else.

Such a standard UIC compartment car has usually only 66 seats in 2nd class, whereas a open coach has 88 seats. For the operating railways that is big difference, especially in countries where rail is having a bit of a renaissance, and thus passenger numbers are increasing. The new DB IC-4 trains have 100 seats per carriage in 2nd class...

Traditional loco hauled trains, using standard rolling stock, are also going away. Railways are increasingly running trains in fixed consists, and most trains ordered now are multiple unit style trains.

I think this has to do with:

• Airplane-style individual seats instead of benches. It is now possible to manufacture a separate comfortable seat for every passenger even in the cheapest class, and compartment can only practically fit benches.
• Movement towards flexible layouts. Instead of having a car of identical 9 compartments, in modern train car you would expect to see: A special area for passengers with disability, kids area, a meeting room, and several rows of chairs with and without tables. It is possible to have mixed-use compartments but it is harder. BTW, a meeting room is practically a compartment, it's just that you usually need to book it as a whole.
• Also, a different approach to privacy. Of course, on average the open layout is more noisy, but it beats being stuck in a compartment with a noisy/unsafe neighbour. To avoid complaints it is easier to average out the experience.
• As an added benefit, modern online booking allows you to choose the precise seat that you are travelling on, so there is more worth to customization and less desire to have all seats equal in all aspects. This compared to the time when you had to stuck your face in a booth to book a ticket and was happy if you got the correct train, not to mention the specific compartment or seat.

First time poster, just one point to add in addition to the other answers, the change between compartments and corridor style rolling stock in the UK was accelerated by the murder of Deborah Linsley, and concerns surrounding the vulnerability of isolated passengers, particularly in stock with compartment access only from the platform.

https://en.wikipedia.org/wiki/Murder_of_Deborah_Linsley

A combination of passenger demand and cost efficiency.

As explained in DavGin's answer already, you can fit more seats and thus more people into a car with an open seat arrangement.

However, that would not have been possible if passengers would not actually prefer that seating. I've travelled a lot by train in Germany where this transition is still ongoing. During the years where most trains (including the IC and ICE trains) had both compartments and open seat areas, the later where always more crowded than the compartments. In fact, for a long time I intentionally booked my seats in compartments and typically had it half full, sometimes the whole thing to myself. Until I once got stuck for several hours with a 7-head family that had booked only 5 seats (small children can sit on your lap, right? until they run around everywhere...) and then I understood why compartments aren't popular.

I also noticed that people in compartments have a considerably higher average age than in the open seating areas. There might be a trend there as well.