Here's my two cents as someone who took the Construction exam and as stated above, spends more time in a field setting with a demanding work schedule. I believe the reason that the passing rate is low is because the exam covers such a broad spectrum of topics which touch on "a little bit of everything" that is contained in the other discipline exams. You've got to be proficient enough in Geotech, WRE, Structural, & Transportation as well as the "Construction" side of the topic. Albeit, there's no reason for a Construction Examinee to delve into the depths of each of these topics like someone who is actually taking these exams, but you need at least a general understanding of the basic principles as they are present in the Depth section. Whereas, if you're taking the Structural exam, you know the questions asking will fall only under than one umbrella. Granted, if I recall correctly, the listed recommended references for Construction pales in comparison to Structural. So I'm sure that's a factor there.
Daily work schedule is definitely an obstacle. I spend 50 hours a week physically at work, and another 10-12 hours commuting per week depending on traffic. There's not much left in the tank when I get home. Then come home to a wife and a small child (at the time, now there's 2 of them) and perform family duties takes whatever is left. Once those are fulfilled, it's study time with whatever you're able to muster up. I'll admit there were a few EET course lectures that I definitely fell asleep during and had to re-watch.
The final reason I think it's so low is because the term "Construction" is so broad. I work for a heavy civil contractor that tackles project that involve just about every type of Civil discipline throughout the projects. So, in my work experience I've been exposed to geotechnical issues/topics (boring logs, pile driving, SOE requirements, etc.), Structural (SOE design, rigging (statics), stresses, concrete, lumber etc), water management, and others. Now, someone else might work for a construction company that deals only with utility installation. Both fall under the umbrella of Construction, but the exposure and experience is totally different. So, that person may say hey I don't do all those other 4 topics but I work in construction so that's the exam for me. In my exam experience, the Depth exam really required "real world" knowledge of topics. So depending on the examinees actual work field and experiences, Construction may not actually be the right test in that respect.
BUT, anyone who is willing to sign up and take this test knows how much of an undertaking it is. I'm sure it's not easy for any field, or walk of life. You have to make the sacrifices necessary to prepare yourself. If the test was easy, everyone with an engineering degree would take it.