Aldarion
Archmage
Basically? Logistics and economy.I'm not a historian. So I am so confused and doubtful of what you're saying, or implying with all this.
The reason is: Forced conscription of commoners happens today, in modern advanced societies. Countries like South Korea and Israel have mandatory service. In the US we still have veterans from Vietnam who were forced into battle.
Is there something that prevented countries from rounding up peasants, spending a month training them on basic pike formations, and sending them into combat? It wouldn't take that much training or all that much equipment.
So color me confused, but why would conscripting the common person into combat be a modern invention?
Now, to be clear here: I am not saying that forced conscription of commoners didn't happen at all, just that it was not a regular thing or a basis of any historical military system. Conscription did happen when one's own city / town / village was under attack... but can you really call it "forced conscription" when the enemy is at the gates and the alternative is getting murdered anyway? You might as well pick up a weapon and go out fighting.
But any sort of campaigning beyond half-a-day's walk away from one's area - and in reality, much closer than that because fields need tending - was simply not economical for anything other than dedicated soldiering class. And in that case, you don't need or want forced conscription, because chances are you will find enough willing recruits keen for plunder.
There are also logistics to consider. When you send an army on a campaign, you also need to feed it. And this also means feeding animals with the army - which means horses, but also the pack animals carrying equipment and food. In Roman army, every contubernion of eight men had a pack animal just for their equipment - and then add the food and so on... numbers get really big, really quickly.
And that is expensive. Food is expensive, animals even more so. Army can also be only so large before it becomes impossible for it to sustain itself from forage when on the enemy territory - and only very centralized, urban states (such as ancient empires and modern states) were able to organize complex system of supply depots required to sustain massive armies of 50 000 or more men.
All of this means is that you have to rely on quality over quantity, because quantity is simply impossible to achieve. And this means using trained soldiers. How you get them, well, now that is a more complex question - you can have full-time professional soldiers, part-time soldiers, mercenaries and so on - but point is, you want your soldiers to be well-trained and as well-equipped as you can afford.
You cannot really use modern societies as a comparative point for medieval society because... again, logistics and economy. In medieval times, it generally took maybe five (and in some areas, even more) families of farmers to support one family of urban dwellers, because food surplus simply wasn't that large. In a modern society, you can have one farming family support five or more families of urban dwellers. It is this advance of agriculture which made it possible to maintain massive armies of conscripts on long campaigns (first such example is probably the French Revolution). And if you are talking about feudal states specifically, those didn't even have the administration that would be necessary to organize the system of conscription you are suggesting, at least not on a scale that would be useful for long wars (Romans could do it, as evidenced by the Punic wars, but even then that was a) only an auxilliary measure - majority of troops were still landowners, b) done in a time of emergency and c) for a very short period of time).