First of all let me make clear that no religion of the world says that you should do only one marriage. Bible, Veda no book talks about it. Now Lets see what Quran says about it? "And if you fear that you will not deal justly with the orphan girls, then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one or those your right hand possesses. That is more suitable that you may not incline [to injustice]."
So in above verse we clearly see that Quran says if you can't do justice between them then only do one marriage but if you can do justice then you can marry up to 4.
Now coming to the Question that why Islam allows to have more than one wife? the answer to this question is very simple and logical. The women population is far more than men in world and situation was also same in the past. there are millions of more women than men in the world.
Now if every man will have only one wife where these millions of women will go? they have two choices. either marry a person who already have a wife or choose dirty path.
More detailed answer of this Question is given by Dr Zakir Naik in this Video:
Source: invocation-rituals.blogspot.com