I agree that few non-STEM leaning students would have MIT as their first choice (even fewer would pick Caltech). They certainly don’t offer nearly the same breadth of course offerings in humanities and social sciences as Harvard or Stanford. Their focus is primarily on STEM after all. However, there’re over 400k STEM graduates each year, and even more students each year intend to choose STEM (because a significant portion of them switch out of STEM before they graduate). The vast majority (well over 95%) of them never applied to MIT/Caltech. So I believe they were deterred by the substantial STEM cores at these schools as @ucbalumnus suggested, or they preferred a less intensive focus on STEM in their undergraduate education.