Salam
Maybe because society at that time valued male opinion more. Even today there are socities where female voices and opinions are not heard or given any importance. These apparitions of angels ( if u r talking about Gabriel, There is evidence that He might not be an angel, read brother Joseph's informative article about this), anyhow these apparitions happened for a reason, to give messages from God, and these messages would be heard by people in a social setting where men and their opinions mattered more. Even today, monotheistic religions like Christianity, Judaism and Islam is propagated by mostly males and female opinion is discarded, u can imagine why God who knows human nature more than we do would have asked angels to take the form of a male person! Even Prophets were males because of this very reason I believe, because revealing a revelation to a woman would not have been accepted as widely in society at that time, and also females were restricted from partaking in many aspects of middle eastern society ( all of the above monotheistic faiths were revealed in middle east) and this situation prevails.
But, whatever I have told u now, is a speculation, the reason behind this phenomenon is not given in the Quran! What I told u is my take on the matter and I might be wrong and I seek forgiveness from Allah for that. These sort of questions do not have answers, except God chose to do so!