I (mu4e’s author) spend a lot of time dealing with e-mail, both professionally and privately. Having an efficient e-mail client is essential. Since none of the existing ones worked the way I wanted, I thought about creating my own.
Emacs is an integral part of my workflow, so it made a lot of sense to use it for e-mail as well. And as I had already written an e-mail search engine (mu), it seemed only logical to use that as a basis.