There is a very common technical reason behind this fact that IRCTC doesn’t allow us to book tickets according to our preference, and the problem behind this is PHYSICS.Booking a seat/seats in a train, is far more different than booking a seat in a theatre crutain. Theater is a Hall, where as trains is a moving object, so according to safety concern is very high in trains. IRCTC ticket booking Software or Applications designed in such a way that it will book tickets in a manner that will distribute the load evenly in a train.
NOW ,Let’s taken an example to make things more clearly : Imagine there are various Classes in a train and the numbered S1, S2 S3… S10, and Every coach there are 1-72 seats. So when someone first books a ticket, software will assign a seat in the middle coach like S5, The middle seat numbered between 20-30, and prefer lower berths. (Railways first fills the lower berths than upper one so as to achieve low center of gravity.)
The software booked seats in such a way that all the coaches have uniform passenger distributions and seats are filled starting from the middle seats like (36) to seats near about the gates i.e :- 1-2 or 71-72 in order from lower to upper breths. Railways (IRCTC) just want to ensure a proper balance that each coach should have for equal load distributions. That ‘s why and when we book a ticket in last and always allotted an upper berth and a seat numbered around 2-3 or 70, Except you are not taking a seat of someone who has cancelled their seat.
If the railways books the tickets randomly, Than A train is a moving to the objects which moves around at the speed of around 100-150Km/Hr on the track. So there are a lots of forces and mechanics acting on the train.
Now Just imagine If the coaches S1, S2, S3 are completely full and S5, S6 are full empty and others are partially full, so when the train taken turns, some coaches are keeping maximum centrifugal force and kept some minimum, and this creates a high chance of the derailment of train. This is a very technical aspect, when brakes are applied there will be different braking forces acting at the time and each of the coaches because of the huge differences in weight of coaches, so that the stability of train becomes an issue again.
Conclusion: There are many forces acting on moving objects/body. Dynamics of a moving body is completely different from a stationary one. So it’s always hard to understand a moving objects. And yes there are technical solutions behind this problems. Like we have developed Double-Decker trains which is an achievement in itself, because it achieves a very high center of gravity force, and for many countries in the world Double-Decker train isn’t possible.
Here, railway ticket booking software is decades oldest version, and at that time railway didn’t have much technology compared to the contemporary time.