Hello Doctor , I Am A 33 Yr Old Female

Hello doctor , I am a 33 yr old female and last week I had a miscarriage coz in 8 week 5 th day of my pregnancy no cardiac activity was dectected n it was confirmed by transvaginal ultrasound also n abdominal usg said that gestational sac size and foetus size was 6 mm which is tat of 6 week and 5 day of pregnancy , and as we have had a usg done at 6 week 5 days in which also no cardiac activity was detected our doc decided for D & C so can u help me why this happened ??? I had mild spotting in 5 week 3 rd day n after tat no spotting n after tat once again spotting happened on 7 week and 4 days.my beta hcg levels on 8 week 6 day was 30,000 & this was my first pregnancy.