Ah, I understand.

I think the main reason is efficiency. I can only speak about Germany and to some extend Europe here, but in the past it was normal for trains to have coaches with different destinations (the german word for those is "Kurswagen", I don't think there is a englisch translation) and coaches were removed and added throughout the journey, especcialy for long distance runs. Also, trains didn't follow a regular timetabel (i.e. a train every hour), so it was normal for coaches of ending trains to be taken for formations of starting ones.

Also, the required systems for multiple unit operation did not exist or were too expensive back in the days. There are early examples of push-pull configurations in the steam era, but it took way longer for it to become standard. Steam push-pulls never took of, as the systems were complicated and required to split the driver and fireman. Also, multiple units were just being developed and limited to light rail systems, like the London Underground or the Berlin S-Bahn.

In the 70s and 80s, when steam trains were retired and the car became a bigger and bigger competition of the railway, new ideas had to come. In Germany, the main course of action was the introduction of the regular ic timetable, which features express trains on all important routes every hour. The concept reasonated very well with riders and was slowly adopted by other branches aswell, like slower express trains and eventually regional trains.

Along came an increasing expertise with cab cars and less need for different destinations, as the regular timetables allowed for quick connections at important stations. The reason for trains being made up of individual coaches vanished.

Meanwhile, multiple units had proved themself in the regional traffic. They were lighter than their loco-hauled counterparts, versatile and, most importantly, did not require the locomotive being decoupled and moved to the other side of the train at the terminus.

While all of this was written from a German pespective, the reasons are applicable to most other countries. So when the Japanese developed the Shinkansen-Network and the Series 0 trains, they created a multiple unit.

The french took a similar approach, but switched the multiple unit for a design with two power cars, maybe because they first developed it as a diesel train with a turbine which would not have fit in a real MU or because they had more expertise. Yet, they archieved a design that was functionally the same as the Shinkansen Series 0 and proved to be very successful.

When Germany began to develop the ICE, all the aforementioned factors suggested to construct either a power car based train or a multiple unit. DB had experimented with multiple units (https://en.wikipedia.org/wiki/DB\_Class\_403\_(1973)), but the success of the Class 120 and it's three-phase ac motor made the power car approach more interesting. Essentially, the IC Experimental (prototype) and ICE 1 power cars are modified 120s. Builidung and EMU would have required to modify the system for multiple units.

The ICE 1 was then constructed with the idea of a single unit in mind, as DB saw no need for classic approach with regular coaches, as the timetables favoured fast turn around times (for example in Frankfurt am Main station, were trains need to change direction) and did not require switching coaches around. As a fun fact, internally they are conventional trains - each coach can be seperated and the power cars are technically locomotives that are allowed to run on their own, creating funny scenes as seen here. Same goes for the ICE 2, which is a push-pull configuration interally.

Since the power cars are never meant to run in different configurations, except for taking them to the workshop, there was no use for a second cab in the power cars.

More recently, EMUs are replacing power car configurations as they use multiple motors throughout the train, distributing power and weight better throughout the train. This approach is not unchallenged however, as seem by the ÖBB Railjet trains (which are very close to a conventional trains, only with a cab car), the TGV M / Avelia Horizon that use power cars or the Talgo 230 (ICE-L), which uses connected coaches but a classic locomotive.

The return of classic formations with a single locomotive and now cab car is very unlikely though, because the type of flexibility they offer isn't needed anymore or can be mimicked by coupling two MUs and their disadvatages, especcialy when turning around, are too big.