The Jewish religion does not leave the two important activities in life, mourning or wailing and rejoicing to chance. The fact is that some people never have anything to mourn about and some people never have anything to rejoice about. Having a special time and place for everyone to mourn and to rejoice makes for unity in the nation.
Every Jew is expected to mourn on the day of destruction of the temple and every Jew is expected to rejoice on the Sabbat day, celebrating the creation of the universe. There is no such thing, in Judaism, as a place where we always mourn, neither is there a place for constant rejoicing. The wall is a place where the nation comes together to mourn on a day specified in Jewish Law for mourning and it is a place for the nation to come together to rejoice for the creation of the universe on the Sabbath, and other days of rejoicing, like Passover. It is not a place designated exclusively for mourning. The people came together at the temple, and the wall, being the only visible remnant of the temple has taken over the function of uniting the Jewish people on festivals of mourning and on festivals of rejoicing.