if men in previous age were allowed to have multiple wives, why not in this age? Similarly to women.

Usually female children take birth more than males. And many men accept the way of renunciation remaining as a naishtika brahmachari for whole life because of that women are greater in number than men.

At the same time it says that women should not be independent. They always should be protected by father, husband then by son.

Because womens' body is considered as weak, we can not physically  protect ourselves so we can not do the duties of a sannyasi like not staying in one place over three days and traveling alone through out the world with no proper cloth and food...

So it is not for us. Women should be protected and they can progress spiritually staying protected at their homes.

But the number of males are less so in previous ages it was allowed to have many wives. Especially for kshatriyas because they are rich. So if one can live life according to religious principles like having sex only for the sake of having children, and support wife and kids financially then one can have any number of wives. (it is for previous ages, nowadays it is forbidden by government)

At the same time all wives should be treated equally not one is a slave other is a Queen. 

Another thing is brahmana if wants to marry first he should marry to a brahmana wife then only to kshatriya then to vaishya then to shudra. 

If kshatriya wants to get married then he can not marry to a brahmani but to a kshatriya then to a vaishya then to a shudrani. So makes 3 wives.

Vishya can marry to a vaihsya then to a shudrani. So makes 2 wives.

Shudra can marry to a shudrani so makes one wife. And it says that in Kali Yuga everyone is a shudra so no chance to marry more than to one wife.

...

These days men can not support a single woman with 3-4 kids what to speak of other wife?

Impossible.