Diagnose w/ectopic pregnancy at 6 weeks.levels @ 6,000 hcg. why cant an ultrasound show an abnormality/sac in fallopian tube at this stage?

Too Early: Early gestational ages are very tough to identify because of the small size of the embryo. That's not to say ultrasound can't diagnose - but if the findings are small enough, ultrasound may not be able to diagnose what is happening. Usually, therefore, radiologists may dictate that "follow-up is recommended to evaluate for intra- or extra-uterine (i.e. ectopic) pregnancy."