Google has server-side blocks that force users to update to a version KitKat cannot run.
Below is a deep review of current options for accessing YouTube on this aging platform. Last Known Stable: Version 14.43.55 (Released Oct 2019). youtube.apk for android 4.4.2
Since the official app often fails to load videos on Android 4.4.2, third-party clients are usually a better choice: Google has server-side blocks that force users to