It is no surprise that cryptography and cryptanalysis require at least a basic, and in most cases an elevated, understanding of mathematics, statistics, and linguistics. Back in the time before cryptography had been developed, such understanding was minimal if anything, and hence it is no surprise that cryptography was as well. As time passed, scholars in these civilizations began to unlock the secrets of cryptanalysis, but it was not until the civilization as a whole had grasped these concepts of math, statistics, and linguistics that cryptanalysis could be really put to good use.
Ever since then, all advanced civilizations have emerged with these basic understandings that we take for granted. Unlike in the time when cryptanalysis was first appearing, the average citizen in most of the modern world can read, can do simple math, and has a basic understanding of statistics. These are the minimal skills required to understand cryptography, and thus it is no surprise that even an amateur of today's world can understand and work with the advanced cryptography that existed back in the earliest civilizations. As we have seen, college students can easily grasp basic substitution ciphers with relative ease, and these basics do not even require the understanding of math and statistics - purely an understanding of linguistics at its most basic form. Thus, the application of frequency analysis seems like an obvious and easy step for a young cryptologist in today's world, whereas when it was first invented, it seemed like a break through that would (and did) change how man viewed cryptography forever.
At the core, an amateur today is as good as a skilled cryptologist hundreds of years ago because today's civilization has a minimum understanding that surpasses the understanding of older civilizations by huge strides. We all have a better understanding of the skills required, which allows us to grasp the advanced methods significantly quicker and at a younger age than was ever imaginable. It is no surprise that a highly advanced method of the past is a commonly used technique today when you consider the differences in understanding between the past and the present.