Why CCNA is difficult.
First, consider the time to prepare before the exam. CCNA is a big set of topics today. My two book CCNA Exam Cert Guide set, if you ignore the pages for front matter and back matter, totals to over 1200 pages. (OK, I do have a rep for being "thorough" in my books, but that's still a lot.) The authorized Cisco courses for CCNA total 10 total days of classroom time - and frankly, I think many people should keep studying after those classes to be ready to pass the 649-802 exam., The rest of the associate and professional level certs essentially cover a single 5-day class's worth of material, with a few exceptions.
The size of coverage results in two underlying issues in my opinion. First and more obvious, the study time required to be ready for the test. Second and less obvious is the forget-before-the-exam issue. If you're diligent, and take 10 hours a week to study CCNA (quite a bit of time per theyek for most folks), you may need 10-15 weeks of such effort to read books, watch videos, discuss in communities, take practice exams, practice subnetting, practice hands-on skills, review, etc. In that time, some of the early reading may just fall out of your memory. Then you need more review, which takes more time, viscous cycle, etc etc.