Basically when you have marraige….it means you publicly appeal to people that i can have now sexual relations with this man or women according to established norms. similar case was in case of concubines….
when it was banned that from now….no slaves are to be made it was autimatically eliminated…
Now you may as well say why sexual relations were allowed with these women…only with their owner…
Let me solve by reverse approach that if it was not permitted:
1)Then you would have to keep …them out of these relationships with any other person…
And if any mishap happened then for the resultant born child you nave to find his father….for living ….food shelter etc
2)if it was kept haram for that time….then you have duty to patronize them during there periods etc…
So it was must that someone has to be made halal for this women to have sexual relationship with her
You would say why not marriage….was established for them…
For marraige everyone knows that ,there are many things to see on…
Like can you marry directly to a women taken from enemies …
Obviosly not…and other factors are as well to be considered firstly
So only thing which could be done in this situation to absorb these in society was to make it halal for her owner….
When it would become clear that now there is no such dangerous indication…obviosly it is recommended to marry her…