First let me preface this by saying I am not Catholic and this is my own personal opinion and I mean no offense.
I personally do not think we, as Christians, are hesitant to say Mary is "blessed among women" but I do think we are hesitant to use the titles "Blessed Virgin Mary" and "Holy Mary, Mother of God" because, to me, although these titles are true, it is placing titles of worship on Mary and she was totally human. and not to be worshiped. When worshipers pray using these titles I feel they are placing Mary in a place above humanity. The angel said "blessed are you among women" not above women. She was just a servant of God like we all are and although she gave birth to God in human form and although He was sinless she was not. I believe she should not be exalted or prayed to because, to me, that is placing her on the same level as Jesus.