There are a variety of reasons someone interested in a career in math, science, or engineering might not consider Caltech.
1. They might not have heard of it. Among top science institutions, Caltech is one of the smaller ones. It makes quite a few more than its proportional share of discoveries, but in total number quite a few others are in the news more.
Also, Caltech is generally only in the news for science and engineering. Some of the larger schools, such as Harvard, are also in the news for business, medicine, history, and other fields, which further raises awareness of them.
2. Caltech has a relatively small undergraduate class. It's about 1/5th the size of MIT, 1/30th the size of UC Berkeley, 1/7th the size of Stanford, 1/7th the size Harvard.
This means it is much less likely that they will know someone who is at Caltech or who went to Caltech. There might have been no prior students from their high school who went there, and so the guidance counsellors might not know much about it and recognize students who would be a great fit.
3. They might think they won't be able to handle the load. Caltech is either the hardest science school in the US, or it is #2 just behind MIT. This can easily scare away people who would actually do fine there.