Self-labelled neo-Luddites and the tech-stressed are searching for phones with fewer features. Industry experts cite precarious profit margins and a wobbly market around this need.
Some of the other comments show that off pretty well. When people say they want a dumb phone they usually want a "dumb" phone that also has X, where X may be their favorite messaging app but it can also be anything else really, like a good camera or support for NFC payments.
+1 For the Light Phone. Owned both their Kickstarter edition and their latest generation, and makes travel, camping, and more easy when I forward my calls/texts. Great battery life with still some creature comforts we have all gotten used to, smart phone wise.