Because everything looks better in black.
Dark mode was one of the most requested features on the YouTube app, this lets you view the app with less bright whites especially when using it at night or in a dark place. Additionally, using dark mode on smartphones with OLED-based displays can help it use less power.
To activate the dark theme, simply head to the settings panel in the YouTube app, and activate the dark theme toggle. Don’t worry if it doesn’t show up in your app, the activation of the feature is server side so it might take a while to roll out to everyone.