They put in quite a lot of time and money to build it and make it work on a ridiculous array of devices running weird embedded OSes.

Open standards hadn't.

Or if you're referring to MMS: insane user cost and insane unreliability, standards mean nothing if the implementations are all trash.

But why didn't email win over WhatsApp? Doesn't email run on all devices?

Not saying it should have, just that I don't really see the difference in functionality.

Email definitely did not. When WhatsApp was beginning its rise to domination, it was on tons of feature phones - mostly just calls, texts, and WhatsApp. Email implies a lot more general internet access than was generally supported or understood by people, since it supports arbitrary data and hosts.