Lots of people, including myself, feel that Evernote is overly complex and not enjoyable to use. I've always wanted a simple app that lets me sync basic text notes across devices. All the ones I've been able to find have something off about them, whether it be interface, lack of sync, complexity, or other annoyance. I find Keep to actually be quite nice, and it fits my needs. I assume there are others for whom this is also true.

Edit: I guess you could say Google's Evernote competitor is just Google Drive itself. If you want text formatting, create an actual Doc. Keep seems to be explicitly for short-form, simple note taking. I think there's a large market for that.