Photoelectric emission

[college-textbooks]

This is a question regarding photoelectric emission when I study Griffiths'
Introduction to Quantum Mechanics
http://www.cocomartini.com/bookstore/in ... ition.html

When frequency of the radiation increases at a constant intensity, the number of electrons emitted per unit time decreases. Why is that so?

p.s. I need an answer ample for Alevel purposes. Not too complex that is.