Around 40% of the population are estimated to suffer from visual stress, where white pages & screens seem to glare and text appears to move around slightly. For others it's just a hindrance in limited lighting and it's not something opticians generally look or test for. If you're dyslexic or dyspraxic (like me) your odds of having it are significantly higher and you'll be in higher need of dark modes or alternative colour schemes.
Visual stress hinders productivity. Evernote, for most, is a productivity app. Not only this, but the lack of such a mode is especially apparent for users who switch between systems, as is encouraged and I imagine many do, making the feature's absence all the more confusing and disappointing.