There are a number of factors, but here are a few that come to mind:
(1) on the medical profession side, many researchers and clinicians don't want to stigmatize all females-- and this is a reasonable thing, because there is this huge cultural stereotype that all females have PMS, and yet actually most females don't show clinically significant changes in mood and thinking across the cycle. So it's reasonable to want to avoid over-diagnosing this kind of thing, and
(2) on the medical profession side, many clinicians just don't know about PMDD, or they think it's just mild PMS that will improve with a bit of self-care. The experimental research validating this condition (as a hormone sensitivity disorder) only came out in 1998, so the research area is young, and there just hasn't been enough education of the medical community yet.
(3) on the sufferer side, there is stigma and shame surrounding behavioral/mental health symptoms (e.g., not wanting to be thought of as "crazy" for receiving treatment);
(4) also on the sufferer side, stigma and shame surrounding the menstrual cycle might prevent them from talking about it (e.g., many females don't want to be seen in a hallway holding a tampon).
So, each of these factors (and probably more) interact to cause low awareness of premenstrual disorders.
Updated 22 December, 2018 by Tory Eisenlohr-Moul, PhD