Exactly as stated in the title. I wonder if it always been that way or it is some modern concept to enforce gender equality?
Obviously not: Swedish is a Germanic language and proto-Germanic was a trigendered language (masculine, feminine, and neuter) as modern German still is.
I don't know when the masculine and feminine gender fused to the common gender in Swedish—in Old Norse in the medieval times the trigender system was still intact. Also, modern Swedish and Danish dialects differ in the number of genders they have (everything from 1 to 3 occurs).
However, it is definitely not a planned language change to enforce gender equality. It developed naturally (the previously distinct endings of the definite forms fell together in one form for masculine and feminine).
EDIT: The introduction of a common (gender-neutral or epicence) pronoun into Swedish in indeed a modern innovation and can be seen as an act of language planning. For details, see the Wikipedia article hen (pronoun)