Final answer:
The spread of secondary education became more widespread due to the growing U.S. population, a shift in school curricula, and the expansion of college opportunities.
Step-by-step explanation:
The spread of secondary education became more widespread for several reasons. One reason is that the U.S. population grew significantly, leading to an increased demand for education.
Additionally, there was a shift in school curricula to a more practical focus, which appealed to more students. Lastly, the expansion of college opportunities and the availability of professional training at land-grant colleges made secondary education more accessible to a wider range of students.