It's not just about Chromebooks, it's the Chromebook in combination with Google Apps for Education which provides Google Mail, Hangouts, Google Docs, and an electronic course environment:
They're cheaper and still good enough. They're popular because that combination is closer to what is desired by the market.
I would say they are better at fulfilling this particular role. A Chromebook meets and exceeds requirements for a computer for many, and with the benefit of low maintenance (it just works), so why would they need what Apple offers.