In Islam, we must obey the commands of Allah and His Messenger, even when the wisdom behind those commands or decrees are unknown to us.
We do not know why we must pray two rak'ahs in the Fajr prayer (rather than three or four, for example), but we know that this is the Decree of Allah and the practice of His Messenger. Likewise, we don't know why the number of wives a man can marry was limited to four, but we know that this was decreed by Allah and proclaimed by His Messenger.
Prior to Islam, there was no limit as to how many women a man could marry. One obvious problem with such system is that a small number of men (typically the richest or the most powerful) could marry all of the marriageable women in the town, and the rest of the men would have to remain single, as they could find no woman to marry.