<p>Most of this reasoning is unconvincing. The difficulty of admission in a graduate program is driven by funding as much as it is driven by competition. </p>
<p>Theoretical disciplines receive less funding, therefore they cannot support as many students. (Also, fewer professors!) In addition, a professor in a theoretical area is only going to want to bother with students who demonstrate enormous capability, because otherwise they will be eat up the scarce funding and not contribute anything (whereas in more applied areas they can still do grunt work in the worst case). </p>
<p>It’s true that humanities programs are more competitive (percentage-wise) than science, as the post you links points out. However, this is because humanities receive even less funding (you can think of them as even more theoretical than theoretical sciences, because there is very little concrete gain). Thus they can only support a few, top students.</p>
<p>In my opinion I think pure math, econ, and certain humanities programs (i.e Philosophy) are the toughest for admissions, followed by CS and Physics, followed by other natural sciences, followed by softer sciences and interdisciplinary fields.</p>